diff --git a/fastapi/encoders.py b/fastapi/encoders.py
index 41c589d49..c4a8bcf76 100644
--- a/fastapi/encoders.py
+++ b/fastapi/encoders.py
@@ -375,19 +375,14 @@ def encode_dict(
         if key not in allowed_keys:
             continue
 
-        # use type() in, instead of isinstance
-        # we don't want to allow subclasses, they should be encoded still
-        if type(key) in primitive_types:
-            encoded_key = key
-        else:
-            encoded_key = encode_value(
-                key,
-                by_alias=by_alias,
-                exclude_unset=exclude_unset,
-                exclude_none=exclude_none,
-                custom_encoder=custom_encoder,
-                sqlalchemy_safe=sqlalchemy_safe,
-            )
+        encoded_key = encode_value(
+            key,
+            by_alias=by_alias,
+            exclude_unset=exclude_unset,
+            exclude_none=exclude_none,
+            custom_encoder=custom_encoder,
+            sqlalchemy_safe=sqlalchemy_safe,
+        )
 
         encoded_value = encode_value(
             value,
diff --git a/tests/test_jsonable_encoder.py b/tests/test_jsonable_encoder.py
index e7e447c1d..6b1b744eb 100644
--- a/tests/test_jsonable_encoder.py
+++ b/tests/test_jsonable_encoder.py
@@ -95,11 +95,23 @@ def test_encode_dict_with_nonprimative_keys():
         def __init__(self, value: str) -> None:
             self.value = value
 
+        def __eq__(self, other) -> bool:
+            return isinstance(other, CustomString) and self.value == other.value
+
+        def __hash__(self):
+            return hash(self.value)
+
     assert jsonable_encoder(
         {CustomString("foo"): "bar"},
         custom_encoder={CustomString: lambda v: v.value}
     ) == {"foo": "bar"}
 
+def test_encode_dict_with_custom_encoder_keys():
+    assert jsonable_encoder(
+        {"foo": "bar"},
+        custom_encoder={str: lambda v: "_" + v}
+    ) == {"_foo": "_bar"}
+
 
 def test_encode_dict_with_sqlalchemy_safe():
     obj = {"_sa_foo": "foo", "bar": "bar"}