diff --git a/mode/services.py b/mode/services.py
index e8850f1..8e01a17 100644
--- a/mode/services.py
+++ b/mode/services.py
@@ -695,7 +695,7 @@ def human_tracebacks(self) -> str:
         )
 
     def _on_future_done(self, fut: asyncio.Future) -> None:
-        self._futures.discard(fut)
+        return self._futures.discard(fut)
 
     def __post_init__(self) -> None:
         """Additional user initialization."""
diff --git a/mode/threads.py b/mode/threads.py
index 2dd0aeb..a11cbbd 100644
--- a/mode/threads.py
+++ b/mode/threads.py
@@ -169,7 +169,7 @@ async def start(self) -> None:
         assert not self._thread_started.is_set()
         self._thread_started.set()
         self._thread_running = asyncio.Future(loop=self.parent_loop)
-        self.add_future(self._keepalive2())
+        fut = self.add_future(self._keepalive2())
         try:
             self._thread = self.Worker(self)
             self._thread.start()
@@ -183,6 +183,7 @@ async def start(self) -> None:
 
                 # wait for thread to be fully started
                 await self._thread_running
+                await fut
         finally:
             self._thread_running = None
 
diff --git a/mode/utils/objects.py b/mode/utils/objects.py
index 8a51580..3745d88 100644
--- a/mode/utils/objects.py
+++ b/mode/utils/objects.py
@@ -29,10 +29,25 @@
     Tuple,
     Type,
     TypeVar,
-    _eval_type,
     cast,
 )
 
+try:
+    from typing import _eval_type  # type: ignore
+except ImportError:
+
+    def _eval_type(t, globalns, localns, recursive_guard=frozenset()):  # type: ignore
+        return t
+
+
+try:
+    from typing import _type_check  # type: ignore
+except ImportError:
+
+    def _type_check(arg, msg, is_argument=True, module=None):  # type: ignore
+        return arg
+
+
 try:
     from typing import _ClassVar  # type: ignore
 except ImportError:  # pragma: no cover
@@ -48,6 +63,25 @@ def _is_class_var(x: Any) -> bool:
         return type(x) is _ClassVar
 
 
+if typing.TYPE_CHECKING:
+
+    class ForwardRef:
+        __forward_arg__: str
+        __forward_evaluated__: bool
+        __forward_value__: Type
+        __forward_code__: Any
+
+        def __init__(self, arg: str, is_argument: bool = True) -> None: ...
+
+else:
+    try:
+        # CPython 3.7
+        from typing import ForwardRef
+    except ImportError:  # pragma: no cover
+        # CPython 3.6
+        from typing import _ForwardRef as ForwardRef
+
+
 __all__ = [
     "FieldMapping",
     "DefaultsMapping",
@@ -374,7 +408,9 @@ def eval_type(
     if isinstance(typ, str):
         typ = ForwardRef(typ)
     if isinstance(typ, ForwardRef):
-        if sys.version_info < (3, 9):
+        if not typ.__forward_evaluated__:
+            typ = _ForwardRef_safe_eval(typ, globalns, localns)
+        elif sys.version_info < (3, 9):
             typ = typ._evaluate(globalns, localns)
         else:
             typ = typ._evaluate(globalns, localns, frozenset())
@@ -384,6 +420,29 @@ def eval_type(
     return alias_types.get(typ, typ)
 
 
+def _ForwardRef_safe_eval(
+    ref: ForwardRef,
+    globalns: Optional[Dict[str, Any]] = None,
+    localns: Optional[Dict[str, Any]] = None,
+) -> Type:
+    # On 3.6/3.7 ForwardRef._evaluate crashes if str references ClassVar
+    if not ref.__forward_evaluated__:
+        if globalns is None and localns is None:
+            globalns = localns = {}
+        elif globalns is None:
+            globalns = localns
+        elif localns is None:
+            localns = globalns
+        val = eval(ref.__forward_code__, globalns, localns)  # noqa: S307
+        if not _is_class_var(val):
+            val = _type_check(
+                val, "Forward references must evaluate to types."
+            )
+        ref.__forward_value__ = val
+        ref.__forward_evaluated__ = True
+    return ref.__forward_value__
+
+
 def _get_globalns(typ: Type) -> Dict[str, Any]:
     return sys.modules[typ.__module__].__dict__