diff --git a/src/py/flwr/client/client_app.py b/src/py/flwr/client/client_app.py
index a8b5a5a85136..1ff374749884 100644
--- a/src/py/flwr/client/client_app.py
+++ b/src/py/flwr/client/client_app.py
@@ -30,25 +30,47 @@
 from .typing import ClientAppCallable
 
 
+def _alert_erroneous_client_fn() -> None:
+    raise ValueError(
+        "A `ClientApp` cannot make use of a `client_fn` that does "
+        "not have a signature in the form: `def client_fn(context: "
+        "Context)`. You can import the `Context` type from `flwr.common`."
+    )
+
+
 def _inspect_maybe_adapt_client_fn_signature(client_fn: ClientFnExt) -> ClientFnExt:
     client_fn_args = inspect.signature(client_fn).parameters
     first_arg = list(client_fn_args.keys())[0]
 
-    if len(client_fn_args) != 1 or client_fn_args[first_arg].annotation is not Context:
-        warn_deprecated_feature(
-            "`client_fn` now expects a signature `def client_fn(context: Context)`."
-            "\The provided `client_fn` has signature: "
-            f"{dict(client_fn_args.items())}"
-        )
-
-        # Wrap depcreated client_fn inside a function with the expected signature
-        def adaptor_fn(context: Context) -> Client:  # pylint: disable=unused-argument
-            # if patition-id is defined, pass it. Else pass node_id that should always
-            # be defined during Context init.
-            cid = context.node_config.get("partition-id", context.node_id)
-            return client_fn(str(cid))  # type: ignore
-
-        return adaptor_fn
+    if len(client_fn_args) == 1:
+
+        first_arg_type = client_fn_args[first_arg].annotation
+
+        if first_arg_type is str:
+            # Warn previous signature for `client_fn` seems to be used
+            warn_deprecated_feature(
+                "`client_fn` now expects a signature `def client_fn(context: Context)`."
+                "The provided `client_fn` has signature: "
+                f"{dict(client_fn_args.items())}. You can import the `Context` type "
+                "from `flwr.common`."
+            )
+
+            # Wrap depcreated client_fn inside a function with the expected signature
+            def adaptor_fn(
+                context: Context,
+            ) -> Client:  # pylint: disable=unused-argument
+                # if patition-id is defined, pass it. Else pass node_id that should
+                # always be defined during Context init.
+                cid = context.node_config.get("partition-id", context.node_id)
+                return client_fn(str(cid))  # type: ignore
+
+            return adaptor_fn
+
+        if first_arg_type is not Context:
+            _alert_erroneous_client_fn()
+
+    else:
+        _alert_erroneous_client_fn()
 
     return client_fn