diff --git a/temporalio/converter.py b/temporalio/converter.py
index 29eb35566..92a5d72e2 100644
--- a/temporalio/converter.py
+++ b/temporalio/converter.py
@@ -1764,9 +1764,7 @@ def value_to_type(
                         elif key_type is type(None):
                             key = {"null": None}[key]
 
-                    # Can't call isinstance if key_type is a newtype
-                    is_newtype = getattr(key_type, "__supertype__", None)
-                    if is_newtype or not isinstance(key, key_type):
+                    if not isinstance(key_type, type) or not isinstance(key, key_type):
                         key = value_to_type(key_type, key, custom_converters)
                 except Exception as err:
                     raise TypeError(
diff --git a/tests/test_converter.py b/tests/test_converter.py
index e274b10d9..a6a292dff 100644
--- a/tests/test_converter.py
+++ b/tests/test_converter.py
@@ -671,3 +671,29 @@ async def test_json_type_converter():
     assert [addr, addr] == (
         await custom_conv.decode([list_payload], [List[ipaddress.IPv4Address]])
     )[0]
+
+
+def test_value_to_type_literal_key():
+    # The type for the dictionary's *key*:
+    KeyHint = Literal[
+        "Key1",
+        "Key2",
+    ]
+
+    # The type for the dictionary's *value* (the inner dict):
+    InnerKeyHint = Literal[
+        "Inner1",
+        "Inner2",
+    ]
+    InnerValueHint = str | int | float | None
+    ValueHint = dict[InnerKeyHint, InnerValueHint]
+
+    # The full type hint for the mapping:
+    hint_with_bug = dict[KeyHint, ValueHint]
+
+    # A value that uses one of the literal keys:
+    value_to_convert = {"Key1": {"Inner1": 123.45, "Inner2": 10}}
+    custom_converters: Sequence[JSONTypeConverter] = []
+
+    # Function executes without error
+    value_to_type(hint_with_bug, value_to_convert, custom_converters)