diff --git a/build_tools/format-diff.sh b/build_tools/format-diff.sh
index 698a6c948..1ed112c2d 100755
--- a/build_tools/format-diff.sh
+++ b/build_tools/format-diff.sh
@@ -119,6 +119,7 @@ then
   # Get the common ancestor with that remote branch. Everything after that
   # common ancestor would be considered the contents of a pull request, so
   # should be relevant for formatting fixes.
+  git merge-base "$FORMAT_UPSTREAM" HEAD
   FORMAT_UPSTREAM_MERGE_BASE="$(git merge-base "$FORMAT_UPSTREAM" HEAD)"
   echo FORMAT_UPSTREAM_MERGE_BASE is $FORMAT_UPSTREAM_MERGE_BASE
   # Get the differences