diff --git a/pybamm/expression_tree/symbol.py b/pybamm/expression_tree/symbol.py
index d3f2dfba2f..c59c69060f 100644
--- a/pybamm/expression_tree/symbol.py
+++ b/pybamm/expression_tree/symbol.py
@@ -615,9 +615,10 @@ def is_constant(self):
     def evaluate_ignoring_errors(self, t=0):
         """
         Evaluates the expression. If a node exists in the tree that cannot be evaluated
-        as a scalar or vector (e.g. Time, Parameter, Variable, StateVector,
-        InputParameter), then None is returned. Otherwise the result of the evaluation
-        is given
+        as a scalar or vector (e.g. Time, Parameter, Variable, StateVector), then None
+        is returned. If there is an InputParameter in the tree then a 1 is returned.
+        Otherwise the result of the evaluation is given.
+
 
         See Also
         --------
diff --git a/tests/unit/test_expression_tree/test_symbol.py b/tests/unit/test_expression_tree/test_symbol.py
index dbff68dd3a..a8462f2559 100644
--- a/tests/unit/test_expression_tree/test_symbol.py
+++ b/tests/unit/test_expression_tree/test_symbol.py
@@ -168,6 +168,13 @@ def test_symbol_evaluation(self):
         with self.assertRaises(NotImplementedError):
             a.evaluate()
 
+    def test_evaluate_ignoring_errors(self):
+        self.assertIsNone(pybamm.t.evaluate_ignoring_errors(t=None))
+        self.assertEqual(pybamm.t.evaluate_ignoring_errors(t=0), 0)
+        self.assertIsNone(pybamm.Parameter("a").evaluate_ignoring_errors())
+        self.assertIsNone(pybamm.StateVector(slice(0,1)).evaluate_ignoring_errors())
+        self.assertEqual(pybamm.InputParameter("a").evaluate_ignoring_errors(), 1)
+
     def test_symbol_is_constant(self):
         a = pybamm.Variable("a")
         self.assertFalse(a.is_constant())