diff --git a/lib/protocol/http/error.rb b/lib/protocol/http/error.rb
index 0e9e05e..8db8c6f 100644
--- a/lib/protocol/http/error.rb
+++ b/lib/protocol/http/error.rb
@@ -26,5 +26,18 @@ def initialize(key)
 			# @attribute [String] key The header key that was duplicated.
 			attr :key
 		end
+		
+		# Raised when an invalid trailer header is encountered in headers.
+		class InvalidTrailerError < Error
+			include BadRequest
+			
+			# @parameter key [String] The trailer key that is invalid.
+			def initialize(key)
+				super("Invalid trailer key: #{key.inspect}")
+			end
+			
+			# @attribute [String] key The trailer key that is invalid.
+			attr :key
+		end
 	end
 end
diff --git a/lib/protocol/http/headers.rb b/lib/protocol/http/headers.rb
index ef1b0ac..21f930e 100644
--- a/lib/protocol/http/headers.rb
+++ b/lib/protocol/http/headers.rb
@@ -191,7 +191,7 @@ def empty?
 			# 	@parameter key [String] The header key.
 			# 	@parameter value [String] The raw header value.
 			def each(&block)
-				@fields.each(&block)
+				self.to_h.each(&block)
 			end
 			
 			# @returns [Boolean] Whether the headers include the specified key.
@@ -279,7 +279,7 @@ def []=(key, value)
 			# @parameter key [String] The header key.
 			# @returns [String | Array | Object] The header value.
 			def [] key
-				to_h[key]
+				self.to_h[key]
 			end
 			
 			# Merge the headers into this instance.
@@ -403,11 +403,12 @@ def delete(key)
 			# @parameter hash [Hash] The hash to merge into.
 			# @parameter key [String] The header key.
 			# @parameter value [String] The raw header value.
+			# @parameter trailer [Boolean] Whether this header is in the trailer section.
 			protected def merge_into(hash, key, value, trailer = @tail)
 				if policy = @policy[key]
 					# Check if we're adding to trailers and this header is allowed:
 					if trailer && !policy.trailer?
-						return false
+						raise InvalidTrailerError, key
 					end
 					
 					if current_value = hash[key]
@@ -418,7 +419,7 @@ def delete(key)
 				else
 					# By default, headers are not allowed in trailers:
 					if trailer
-						return false
+						raise InvalidTrailerError, key
 					end
 					
 					if hash.key?(key)
@@ -431,6 +432,8 @@ def delete(key)
 			
 			# Compute a hash table of headers, where the keys are normalized to lower case and the values are normalized according to the policy for that header.
 			#
+			# This will enforce policy rules, such as merging multiple headers into arrays, or raising errors for duplicate headers.
+			# 
 			# @returns [Hash] A hash table of `{key, value}` pairs.
 			def to_h
 				unless @indexed
@@ -461,7 +464,7 @@ def inspect
 			def == other
 				case other
 				when Hash
-					to_h == other
+					self.to_h == other
 				when Headers
 					@fields == other.fields
 				else
diff --git a/releases.md b/releases.md
index 78672d8..2c4dbd4 100644
--- a/releases.md
+++ b/releases.md
@@ -1,5 +1,10 @@
 # Releases
 
+## Unreleased
+
+  - Always use `#parse` when parsing header values from strings to ensure proper normalization and validation.
+  - Introduce `Protocol::HTTP::InvalidTrailerError` which is raised when a trailer header is not allowed by the current policy.
+
 ## v0.56.0
 
   - Introduce `Header::*.parse(value)` which parses a raw header value string into a header instance.
diff --git a/test/protocol/http/headers.rb b/test/protocol/http/headers.rb
index 8b73d10..f880dee 100644
--- a/test/protocol/http/headers.rb
+++ b/test/protocol/http/headers.rb
@@ -153,7 +153,7 @@
 		end
 		
 		it "can enumerate fields" do
-			headers.each.with_index do |field, index|
+			headers.fields.each_with_index do |field, index|
 				expect(field).to be == fields[index]
 			end
 		end
@@ -343,7 +343,7 @@
 				it "can't add a #{key.inspect} header in the trailer", unique: key do
 					trailer = headers.trailer!
 					headers.add(key, "example")
-					expect(headers).not.to be(:include?, key)
+					expect{headers.to_h}.to raise_exception(Protocol::HTTP::InvalidTrailerError)
 				end
 			end
 		end