diff --git a/ocpp/charge_point.py b/ocpp/charge_point.py
index bdd75ae6e..78f56549e 100644
--- a/ocpp/charge_point.py
+++ b/ocpp/charge_point.py
@@ -165,7 +165,7 @@ async def _handle_call(self, msg):
 
         First the '_on_action' hook is executed and its response is returned to
         the client. If there is no '_on_action' hook for Action in the message
-        a CallError with a NotImplemtendError is returned.
+        a CallError with a NotSupportedError is returned.
 
         Next the '_after_action' hook is executed.
 
diff --git a/ocpp/messages.py b/ocpp/messages.py
index e14eebc29..5d6012869 100644
--- a/ocpp/messages.py
+++ b/ocpp/messages.py
@@ -437,8 +437,8 @@ def to_exception(self):
                 )
 
         raise UnknownCallErrorCodeError(
-            "Error code '%s' is not defined by the" " OCPP specification",
-            self.error_code,
+            f"Error code '{self.error_code}' is not defined by the"
+            " OCPP specification"
         )
 
     def __repr__(self):