diff --git a/NAMESPACE b/NAMESPACE
index 7ae7a68..6d44595 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -2,6 +2,7 @@
 
 export(check_container_class)
 export(check_envvar)
+export(check_that)
 export(get_auth_token)
 export(get_container)
 export(list_container_names)
diff --git a/R/azkit_helpers.R b/R/azkit_helpers.R
index 8003217..550bca7 100644
--- a/R/azkit_helpers.R
+++ b/R/azkit_helpers.R
@@ -1,3 +1,29 @@
+#' An alternative to stopifnot/assert_that etc
+#'
+#' If the predicate function is true of `x` then `x` is returned. Otherwise,
+#'  an error is thrown with a custom `message`.
+#'
+#' @param x The object to be checked
+#' @param predicate The predicate function used to check `x`
+#' @param message A custom error message, as a string. Will be shown to the
+#'  user if the predicate check does not succeed. Can include `glue`d variables
+#'  and `{cli}` semantic markup.
+#' @param pf Set as [parent.frame()] so variables in the caller environment can
+#'  be used in the custom error message.
+#' @seealso [check_vec]
+#' @export
+check_that <- function(x, predicate, message, pf = parent.frame()) {
+  if (predicate(x)) {
+    x
+  } else {
+    cli::cli_abort(message, call = rlang::caller_call(), .envir = pf)
+  }
+}
+
+
+#' @keywords internal
+ct_error_msg <- \(text) paste0("{.fn check_that}: ", text)
+
 #' An alternative to stopifnot/assert_that etc
 #'
 #' This function makes it easy to use the `{purrr}` functions `every()`,
@@ -6,10 +32,9 @@
 #' Not suitable for checking if `length(x) == 1` as it will check vectors
 #' element-wise, so will potentially return `TRUE` even if `length(x) > 1`
 #'
-#' @param x The object to be checked
 #' @param predicate The predicate function used to check elements of `x`
 #' @param message A custom error message, as a string. Will be shown to the
-#'  user if the predicate check does not succeed. Can include `glue` variables
+#'  user if the predicate check does not succeed. Can include `glue`d variables
 #'  and `{cli}` semantic markup. Variable values will be searched for in the
 #'  environment of the caller function (not in the environment of `check_vec()`
 #'  ). This makes it easier to include informative values in the message.
@@ -19,8 +44,7 @@
 #'  predicate. "none" can be used to generate an inverse predicate, or the
 #'  situation where success means that none of the elements of x satisfies the
 #'  predicate. "some" is unlikely to be useful often, but it is available.
-#' @param pf Set as [parent.frame] so variables in the caller environment can
-#'  be used in the custom error message.
+#' @inheritParams check_that
 #' @seealso [check_scalar_type()]
 #' @keywords internal
 check_vec <- function(
@@ -51,8 +75,8 @@ cv_error_msg <- \(text) paste0("{.fn check_vec}: ", text)
 #' Possible values for the `type` parameter are: "character", "logical", "list",
 #'  "integer", "double", "string", "bool", "bytes", "raw", "vector", "complex".
 #  Supplying "string" or "bool" will additionally check that `x` is not missing.
-#' @seealso [check_vec()]
-#' @inheritParams check_vec
+#' @seealso [check_that]
+#' @inheritParams check_that
 #' @param type A string defining the R object type that `x` is checked to be
 #' @keywords internal
 check_scalar_type <- function(
@@ -88,6 +112,32 @@ check_scalar_type <- function(
 cst_error_msg <- \(text) paste0("{.fn check_scalar_type}: ", text)
 
 
+#' Check if a supplied non-NULL value is a string with >0 characters
+#'
+#' Will error if x is equal to `""`, or if it is otherwise missing or invalid.
+#' With the exception that if x is NULL, then NULL will be passed through.
+#' @inheritParams check_that
+#' @param message A custom error message, as a string. Will be shown to the
+#'  user if the check does not pass. Can include `glue` variables and `{cli}`
+#'  semantic markup. Variable values will be searched for in the environment of
+#'  the caller function (not in the environment of `check_nzchar()`). This
+#'  makes it easier to include informative values in the message.
+#' @keywords internal
+check_nzchar <- function(x, message, pf = parent.frame()) {
+  if (is.null(x)) {
+    NULL
+  }
+  cnz <- "check_nzchar" # nolint
+  check_scalar_type(x, "string", "{.fn {cnz}}: {.var x} is not a string")
+  if (nzchar(x)) {
+    x
+  } else {
+    message <- paste0("{.fn {cnz}}: ", message)
+    cli::cli_abort(message, call = rlang::caller_call(), .envir = pf)
+  }
+}
+
+
 #' grepl a glued regex
 #'
 #' Use \{glue\} expressions in grepl (and put the arguments the right way round)
diff --git a/R/get_container.R b/R/get_container.R
index f090401..edc7331 100644
--- a/R/get_container.R
+++ b/R/get_container.R
@@ -1,23 +1,27 @@
 #' Get Azure storage container
 #'
-#' Use [list_container_names] to see a list of available containers
+#' The environment variable "AZ_STORAGE_EP" must be set. This provides the URL
+#'  for the default Azure storage endpoint.
+#' Use [list_container_names] to get a list of available container names.
 #'
 #' @param container_name Name of the container as a string. `NULL` by default,
 #'  which means the function will look instead for a container name stored in
 #'  the environment variable "AZ_CONTAINER"
-#' @param ... arguments to be passed through to [get_auth_token()]
+#' @param ... arguments to be passed through to [get_auth_token]
 #' @returns An Azure blob container (list object of class "blob_container")
 #' @export
 get_container <- function(container_name = NULL, ...) {
-  cst_msg <- cst_error_msg("{.var container_name} must be a string")
-  container_name <- (container_name %||% check_envvar("AZ_CONTAINER")) |>
-    check_scalar_type("character", cst_msg)
+  msg <- glue::glue(
+    "{.var container_name} is empty. ",
+    "Did you forget to set an environment variable?"
+  )
+  cont_nm <- check_nzchar(container_name, msg) %||% check_envvar("AZ_CONTAINER")
   token <- get_auth_token(...)
   endpoint <- get_default_endpoint(token)
   container_names <- list_container_names(token)
-  not_found_msg <- cv_error_msg("Container {.val {container_name}} not found")
-  container_name |>
-    check_vec(\(x) x %in% container_names, not_found_msg) |>
+  not_found_msg <- ct_error_msg("Container {.val {cont_nm}} not found")
+  cont_nm |>
+    check_that(\(x) x %in% container_names, not_found_msg) |>
     AzureStor::blob_container(endpoint = endpoint)
 }
 
@@ -59,6 +63,6 @@ get_default_endpoint <- function(token) {
 #' @returns the value of the environment variable named in `x`
 #' @export
 check_envvar <- function(x) {
-  cst_msg <- cst_error_msg("{.envvar {x}} is not set")
+  cst_msg <- cst_error_msg("The environment variable {.envvar {x}} is not set")
   check_scalar_type(Sys.getenv(x, NA_character_), "string", cst_msg)
 }
diff --git a/R/list_files.R b/R/list_files.R
index a33ed39..cfff3f3 100644
--- a/R/list_files.R
+++ b/R/list_files.R
@@ -31,8 +31,8 @@
 list_files <- function(container, path = "", ext = "", recursive = TRUE) {
   stopifnot(rlang::is_character(c(path, ext), 2))
   stopifnot(rlang::is_bool(recursive))
-  pnf_msg <- cv_error_msg("Path {.val {path}} not found")
-  check_vec(path, \(x) AzureStor::blob_dir_exists(container, x), pnf_msg)
+  pnf_msg <- ct_error_msg("Path {.val {path}} not found")
+  check_that(path, \(x) AzureStor::blob_dir_exists(container, x), pnf_msg)
 
   tbl <- AzureStor::list_blobs(container, path, recursive = recursive)
   if (nrow(tbl) > 0) {
diff --git a/R/read_azure_files.R b/R/read_azure_files.R
index a5806e6..201ecda 100644
--- a/R/read_azure_files.R
+++ b/R/read_azure_files.R
@@ -14,7 +14,7 @@
 #'  being read. Useful for checking the function is doing what is expected, but
 #'  can be turned off with `FALSE`. Can be set persistently with the option
 #'  "azkit.info". If `NULL` then it will default to the value of
-#'  [rlang::is_interactive] (ie `TRUE` for interactive sessions).
+#'  [rlang::is_interactive] (that is, `TRUE` for interactive sessions).
 #' @param ... optional arguments to be passed through to [arrow::read_parquet]
 #' @returns A tibble
 #' @examples \dontrun{
@@ -139,12 +139,10 @@ check_blob_exists <- function(container, file, ext, info, path) {
     dplyr::filter(dplyr::if_any("name", \(x) x == {{ file_path }})) |>
     dplyr::pull("name")
 
-  if (length(filepath_out) == 0) {
-    cli::cli_abort("no matching {ext} file found")
-  }
-  if (length(filepath_out) > 1) {
-    cli::cli_abort("multiple matching {ext} files found")
-  }
+  msg1 <- ct_error_msg("no matching {ext} file found")
+  msg2 <- cst_error_msg("multiple matching {ext} files found")
+  check_that(filepath_out, \(x) length(x) > 0, msg1) # check length > 0
+  check_scalar_type(filepath_out, "character", msg2) # check length == 1
 
   info_option <- getOption("azkit.info")
   stopifnot(rlang::is_scalar_logical(info) || is.null(info))
diff --git a/man/check_blob_exists.Rd b/man/check_blob_exists.Rd
index d513fe9..472231a 100644
--- a/man/check_blob_exists.Rd
+++ b/man/check_blob_exists.Rd
@@ -19,7 +19,7 @@ will error if multiple files are somehow matched.}
 being read. Useful for checking the function is doing what is expected, but
 can be turned off with \code{FALSE}. Can be set persistently with the option
 "azkit.info". If \code{NULL} then it will default to the value of
-\link[rlang:is_interactive]{rlang::is_interactive} (ie \code{TRUE} for interactive sessions).}
+\link[rlang:is_interactive]{rlang::is_interactive} (that is, \code{TRUE} for interactive sessions).}
 
 \item{path}{The path to the directory where \code{file} is located, as a string.
 Only needed if \code{file} does not already contain its full path. If file is
diff --git a/man/check_nzchar.Rd b/man/check_nzchar.Rd
new file mode 100644
index 0000000..72ae9b6
--- /dev/null
+++ b/man/check_nzchar.Rd
@@ -0,0 +1,25 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/azkit_helpers.R
+\name{check_nzchar}
+\alias{check_nzchar}
+\title{Check if a supplied non-NULL value is a string with >0 characters}
+\usage{
+check_nzchar(x, message, pf = parent.frame())
+}
+\arguments{
+\item{x}{The object to be checked}
+
+\item{message}{A custom error message, as a string. Will be shown to the
+user if the check does not pass. Can include \code{glue} variables and \code{{cli}}
+semantic markup. Variable values will be searched for in the environment of
+the caller function (not in the environment of \code{check_nzchar()}). This
+makes it easier to include informative values in the message.}
+
+\item{pf}{Set as \code{\link[=parent.frame]{parent.frame()}} so variables in the caller environment can
+be used in the custom error message.}
+}
+\description{
+Will error if x is equal to \code{""}, or if it is otherwise missing or invalid.
+With the exception that if x is NULL, then NULL will be passed through.
+}
+\keyword{internal}
diff --git a/man/check_scalar_type.Rd b/man/check_scalar_type.Rd
index c3f6083..ef9f097 100644
--- a/man/check_scalar_type.Rd
+++ b/man/check_scalar_type.Rd
@@ -12,10 +12,8 @@ check_scalar_type(x, type, message, pf = parent.frame())
 \item{type}{A string defining the R object type that \code{x} is checked to be}
 
 \item{message}{A custom error message, as a string. Will be shown to the
-user if the predicate check does not succeed. Can include \code{glue} variables
-and \code{{cli}} semantic markup. Variable values will be searched for in the
-environment of the caller function (not in the environment of \code{check_vec()}
-). This makes it easier to include informative values in the message.}
+user if the predicate check does not succeed. Can include \code{glue}d variables
+and \code{{cli}} semantic markup.}
 
 \item{pf}{Set as \link{parent.frame} so variables in the caller environment can
 be used in the custom error message.}
@@ -28,6 +26,6 @@ Possible values for the \code{type} parameter are: "character", "logical", "list
 "integer", "double", "string", "bool", "bytes", "raw", "vector", "complex".
 }
 \seealso{
-\code{\link[=check_vec]{check_vec()}}
+\link{check_that}
 }
 \keyword{internal}
diff --git a/man/check_that.Rd b/man/check_that.Rd
new file mode 100644
index 0000000..9140faf
--- /dev/null
+++ b/man/check_that.Rd
@@ -0,0 +1,27 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/azkit_helpers.R
+\name{check_that}
+\alias{check_that}
+\title{An alternative to stopifnot/assert_that etc}
+\usage{
+check_that(x, predicate, message, pf = parent.frame())
+}
+\arguments{
+\item{x}{The object to be checked}
+
+\item{predicate}{The predicate function used to check \code{x}}
+
+\item{message}{A custom error message, as a string. Will be shown to the
+user if the predicate check does not succeed. Can include \code{glue}d variables
+and \code{{cli}} semantic markup.}
+
+\item{pf}{Set as \code{\link[=parent.frame]{parent.frame()}} so variables in the caller environment can
+be used in the custom error message.}
+}
+\description{
+If the predicate function is true of \code{x} then \code{x} is returned. Otherwise,
+an error is thrown with a custom \code{message}.
+}
+\seealso{
+\link{check_vec}
+}
diff --git a/man/check_vec.Rd b/man/check_vec.Rd
index d72ab8e..ea4698b 100644
--- a/man/check_vec.Rd
+++ b/man/check_vec.Rd
@@ -18,7 +18,7 @@ check_vec(
 \item{predicate}{The predicate function used to check elements of \code{x}}
 
 \item{message}{A custom error message, as a string. Will be shown to the
-user if the predicate check does not succeed. Can include \code{glue} variables
+user if the predicate check does not succeed. Can include \code{glue}d variables
 and \code{{cli}} semantic markup. Variable values will be searched for in the
 environment of the caller function (not in the environment of \code{check_vec()}
 ). This makes it easier to include informative values in the message.}
diff --git a/man/get_container.Rd b/man/get_container.Rd
index 3d378ae..7fe762a 100644
--- a/man/get_container.Rd
+++ b/man/get_container.Rd
@@ -11,11 +11,13 @@ get_container(container_name = NULL, ...)
 which means the function will look instead for a container name stored in
 the environment variable "AZ_CONTAINER"}
 
-\item{...}{arguments to be passed through to \code{\link[=get_auth_token]{get_auth_token()}}}
+\item{...}{arguments to be passed through to \link{get_auth_token}}
 }
 \value{
 An Azure blob container (list object of class "blob_container")
 }
 \description{
-Use \link{list_container_names} to see a list of available containers
+The environment variable "AZ_STORAGE_EP" must be set. This provides the URL
+for the default Azure storage endpoint.
+Use \link{list_container_names} to get a list of available container names.
 }
diff --git a/tests/testthat/test-read_azure_files.R b/tests/testthat/test-read_azure_files.R
index 06535c8..1f9e0fc 100644
--- a/tests/testthat/test-read_azure_files.R
+++ b/tests/testthat/test-read_azure_files.R
@@ -236,7 +236,7 @@ test_that("tdd of check_blob_exists", {
         dplyr::pull("name")
       stop_msg1 <- glue::glue("no matching {file_ext} file found")
       stop_msg2 <- glue::glue("multiple matching {file_ext} files found")
-      check_vec(filepath, rlang::is_character, stop_msg1) # check length > 0
+      check_that(filepath, \(x) length(x) > 0, stop_msg1) # check length > 0
       check_scalar_type(filepath, "character", stop_msg2) # check length == 1
     }
     expect_error(check_blob_exists(support_container, "unmatched"), "matching")
@@ -255,7 +255,7 @@ test_that("tdd of check_blob_exists", {
         dplyr::pull("name")
       stop_msg1 <- glue::glue("no matching {file_ext} file found")
       stop_msg2 <- glue::glue("multiple matching {file_ext} files found")
-      check_vec(filepath, rlang::is_character, stop_msg1) # check length > 0
+      check_that(filepath, \(x) length(x) > 0, stop_msg1) # check length > 0
       check_scalar_type(filepath, "character", stop_msg2) # check length == 1
       filepath
     }