Remove non specified return

src/zfitwrapper/BaseModel.py

@@ -148,6 +148,9 @@ class BaseModel(ABC):
                 if parameter.fraction:
                     return self.get_modelparameter(parametername)
 
+            else:
+                raise KeyError("No fraction parameter can be found")
+
     def add_constraint(
         self, parameter: str, constraint: str = "GaussianConstraint", **kwargs
     ) -> None: