diff --git a/fastapi/encoders.py b/fastapi/encoders.py
index 451ea0760..8182e2277 100644
--- a/fastapi/encoders.py
+++ b/fastapi/encoders.py
@@ -219,7 +219,7 @@ def jsonable_encoder(
         if not PYDANTIC_V2:
             encoders = getattr(obj.__config__, "json_encoders", {})  # type: ignore[attr-defined]
             if custom_encoder:
-                encoders.update(custom_encoder)
+                encoders = {**encoders, **custom_encoder}
         obj_dict = _model_dump(
             obj,
             mode="json",
diff --git a/tests/test_jsonable_encoder.py b/tests/test_jsonable_encoder.py
index 1906d6bf1..447c5b4d6 100644
--- a/tests/test_jsonable_encoder.py
+++ b/tests/test_jsonable_encoder.py
@@ -216,9 +216,12 @@ def test_custom_encoders():
     instance = MyModel(dt_field=safe_datetime.now())
 
     encoded_instance = jsonable_encoder(
-        instance, custom_encoder={safe_datetime: lambda o: o.isoformat()}
+        instance, custom_encoder={safe_datetime: lambda o: o.strftime("%H:%M:%S")}
     )
-    assert encoded_instance["dt_field"] == instance.dt_field.isoformat()
+    assert encoded_instance["dt_field"] == instance.dt_field.strftime("%H:%M:%S")
+
+    encoded_instance2 = jsonable_encoder(instance)
+    assert encoded_instance2["dt_field"] == instance.dt_field.isoformat()
 
 
 def test_custom_enum_encoders():