diff --git a/symfit/core/fit.py b/symfit/core/fit.py
index <HASH>..<HASH> 100644
--- a/symfit/core/fit.py
+++ b/symfit/core/fit.py
@@ -729,7 +729,7 @@ class Constraint(Model):
         # raise Exception(model)
         if isinstance(constraint, Relational):
             self.constraint_type = type(constraint)
-            if isinstance(model, Model):
+            if isinstance(model, BaseModel):
                 self.model = model
             else:
                 raise TypeError('The model argument must be of type Model.')
@@ -2269,4 +2269,4 @@ class ODEModel(CallableModel):
         bound_arguments = self.__signature__.bind(*args, **kwargs)
         Ans = namedtuple('Ans', [var.name for var in self])
         ans = Ans(*self.eval_components(**bound_arguments.arguments))
-        return ans
\ No newline at end of file
+        return ans
Constraint object work with all models now