diff --git a/matemat/util/test/test_currency_format.py b/matemat/util/test/test_currency_format.py
index 882ddac..b2f6eb5 100644
--- a/matemat/util/test/test_currency_format.py
+++ b/matemat/util/test/test_currency_format.py
@@ -131,3 +131,9 @@ class TestCurrencyFormat(unittest.TestCase):
             parse_chf('13,37')
         with self.assertRaises(ValueError):
             parse_chf('CHF 13,37')
+
+    def test_parse_frac_negative(self):
+        with self.assertRaises(ValueError):
+            parse_chf('13.-7')
+        with self.assertRaises(ValueError):
+            parse_chf('CHF 13.-7')