diff --git a/dropbox/stone_validators.py b/dropbox/stone_validators.py
index 372d0a4d..c8534b84 100644
--- a/dropbox/stone_validators.py
+++ b/dropbox/stone_validators.py
@@ -305,7 +305,7 @@ def validate(self, val):
             try:
                 val = val.decode('utf-8')
             except UnicodeDecodeError:
-                raise ValidationError("'%s' was not valid utf-8")
+                raise ValidationError("'%s' was not valid utf-8" % val)
 
         if self.max_length is not None and len(val) > self.max_length:
             raise ValidationError("'%s' must be at most %d characters, got %d"