diff --git a/trident/roman.py b/trident/roman.py
index 3f6234582d..909d185a20 100644
--- a/trident/roman.py
+++ b/trident/roman.py
@@ -100,7 +100,7 @@ def from_roman(s):
     if not s:
         raise InvalidRomanNumeralError('Input can not be blank')
     if not romanNumeralPattern.search(s):
-        raise InvalidRomanNumeralError('Invalid Roman numeral: {s}'.format(s))
+        raise InvalidRomanNumeralError('Invalid Roman numeral: {0}'.format(s))
 
     result = 0
     index = 0