diff --git a/pybamm/discretisations/discretisation.py b/pybamm/discretisations/discretisation.py
index abace025f3..b74ac08a72 100644
--- a/pybamm/discretisations/discretisation.py
+++ b/pybamm/discretisations/discretisation.py
@@ -994,8 +994,8 @@ def check_initial_conditions(self, model):
         """Check initial conditions are a numpy array"""
         # Individual
         for var, eqn in model.initial_conditions.items():
-            assert (
-                type(eqn.evaluate(t=0, u="shape test")) is np.ndarray
+            assert isinstance(
+                eqn.evaluate(t=0, u="shape test"), np.ndarray
             ), pybamm.ModelError(
                 """
                 initial_conditions must be numpy array after discretisation but they are