diff --git a/tailbone/forms/widgets.py b/tailbone/forms/widgets.py
index 7c89fa7d..b90f9dda 100644
--- a/tailbone/forms/widgets.py
+++ b/tailbone/forms/widgets.py
@@ -79,7 +79,10 @@ class PercentInputWidget(dfwidget.TextInputWidget):
         if pstruct is colander.null:
             return colander.null
         # convert "human-friendly" value to "traditional"
-        value = decimal.Decimal(pstruct)
+        try:
+            value = decimal.Decimal(pstruct)
+        except decimal.InvalidOperation:
+            raise colander.Invalid(field.schema, "Invalid decimal string: {}".format(pstruct))
         value = value.quantize(decimal.Decimal('0.00001'))
         value /= 100
         return six.text_type(value)