diff --git a/src/py/flwr/common/metricsrecord.py b/src/py/flwr/common/metricsrecord.py
index 76ab41d1ad30..aa683ff7a2ca 100644
--- a/src/py/flwr/common/metricsrecord.py
+++ b/src/py/flwr/common/metricsrecord.py
@@ -32,7 +32,7 @@ def add_metrics(self, metrics_dict: Dict[str, Union[Scalar, ScalarList]]) -> Non
         This not implemented as a constructor so we can cleanly create and empyt
         MetricsRecord object.
         """
-        if any(not (k, str) for k in metrics_dict.keys()):
+        if any(not isinstance(k, str) for k in metrics_dict.keys()):
             raise TypeError(f"Not all keys are of valide type. Expected {str}")
 
         def is_valid(value: Scalar) -> None: