diff --git a/src/main/py/read_math_function.py b/src/main/py/read_math_function.py
index 0b2cdd9..00a9327 100644
--- a/src/main/py/read_math_function.py
+++ b/src/main/py/read_math_function.py
@@ -4,6 +4,8 @@ def read_constant(type, value):
     if is_string(value):
         if is_allowed_string(type, value):
             return calculate_string(value)
+        else:
+            return False
     return value
 
 
diff --git a/src/test/py/test_read_math_function.py b/src/test/py/test_read_math_function.py
index b8fb7d1..0fe03d2 100644
--- a/src/test/py/test_read_math_function.py
+++ b/src/test/py/test_read_math_function.py
@@ -21,6 +21,8 @@ class read_function(unittest.TestCase):
         self.assertEqual(calculate_string("42^(2)+1-3*4/3"), 1761)
     def test_constant_calculate_string_decision(self):
         self.assertEqual(read_constant("c", "42^(2)+1-3*4/3"), 1761)
+    def test_constant_string_not_allowed_decision(self):
+        self.assertEqual(read_constant("c", "42^(2)+1-3*4/3x"), False)
 
 if __name__ == '__main__':
     unittest.main()