diff --git a/frappy/params.py b/frappy/params.py
index a414b08..c1f7fc0 100644
--- a/frappy/params.py
+++ b/frappy/params.py
@@ -233,13 +233,13 @@ class Parameter(Accessible):
         try:
             return instance.parameters[self.name].value
         except KeyError:
-            raise ProgrammingError(f'optional parameter {self.name} it is not implemented') from None
+            raise ProgrammingError(f'optional parameter {self.name} is not implemented') from None
 
     def __set__(self, obj, value):
         try:
             obj.announceUpdate(self.name, value)
         except KeyError:
-            raise ProgrammingError(f'optional parameter {self.name} it is not implemented') from None
+            raise ProgrammingError(f'optional parameter {self.name} is not implemented') from None
 
     def __set_name__(self, owner, name):
         self.name = name