diff --git a/festim/boundary_conditions/dirichlet_bc.py b/festim/boundary_conditions/dirichlet_bc.py
index efe4544a8..b1ca05361 100644
--- a/festim/boundary_conditions/dirichlet_bc.py
+++ b/festim/boundary_conditions/dirichlet_bc.py
@@ -99,8 +99,10 @@ def create_value(
 
             if "t" in arguments and "x" not in arguments and "T" not in arguments:
                 # only t is an argument
-                if isinstance(self.value(t=float(t)), Operator):
-                    raise ValueError("wrong type for temperature")
+                if not isinstance(self.value(t=float(t)), (float, int)):
+                    raise ValueError(
+                        f"self.value should return a float or an int, not {type(self.value(t=float(t)))} "
+                    )
                 self.value_fenics = F.as_fenics_constant(
                     mesh=mesh, value=self.value(t=float(t))
                 )
diff --git a/festim/hydrogen_transport_problem.py b/festim/hydrogen_transport_problem.py
index 7ff3ddd46..6017307d7 100644
--- a/festim/hydrogen_transport_problem.py
+++ b/festim/hydrogen_transport_problem.py
@@ -197,8 +197,10 @@ def define_temperature(self):
         elif callable(self.temperature):
             arguments = self.temperature.__code__.co_varnames
             if "t" in arguments and "x" not in arguments and "T" not in arguments:
-                if isinstance(self.temperature(t=float(self.t)), Operator):
-                    raise ValueError("wrong type for temperature")
+                if not isinstance(self.temperature(t=float(self.t)), (float, int)):
+                    raise ValueError(
+                        f"self.temperature should return a float or an int, not {type(self.temperature(t=float(self.t)))} "
+                    )
                 # only t is an argument
                 self.temperature_fenics = F.as_fenics_constant(
                     mesh=self.mesh.mesh, value=self.temperature(t=float(self.t))
diff --git a/test/test_dirichlet_bc.py b/test/test_dirichlet_bc.py
index e0bbeaf92..ea17569b7 100644
--- a/test/test_dirichlet_bc.py
+++ b/test/test_dirichlet_bc.py
@@ -304,7 +304,9 @@ def test_define_value_error_if_ufl_conditional_t_only(value):
 
     t = fem.Constant(mesh, 0.0)
 
-    with pytest.raises(ValueError, match="wrong type for temperature"):
+    with pytest.raises(
+        ValueError, match="self.value should return a float or an int, not "
+    ):
         bc.create_value(mesh=mesh, function_space=None, temperature=None, t=t)
 
 
diff --git a/test/test_h_transport_problem.py b/test/test_h_transport_problem.py
index 53b308845..09680016e 100644
--- a/test/test_h_transport_problem.py
+++ b/test/test_h_transport_problem.py
@@ -116,7 +116,9 @@ def test_define_temperature_error_if_ufl_conditional_t_only(input):
 
     my_model.temperature = input
 
-    with pytest.raises(ValueError, match="wrong type for temperature"):
+    with pytest.raises(
+        ValueError, match="self.temperature should return a float or an int, not "
+    ):
         my_model.define_temperature()