diff --git a/polly/utils/check_format.sh b/polly/utils/check_format.sh
index 94a14a1873f2..b7b68bf9fce1 100755
--- a/polly/utils/check_format.sh
+++ b/polly/utils/check_format.sh
@@ -4,7 +4,7 @@ OK=0
 
 for ARG in "$@"
   do
-    clang-format $1 | diff $1 -
+    clang-format $1 | diff -u $1 -
 
     if [[ $? -eq 1 ]]; then
       OK=1