diff --git a/diff-sha.sh b/diff-sha.sh
index f2ba2b87..2f95d203 100644
--- a/diff-sha.sh
+++ b/diff-sha.sh
@@ -155,9 +155,11 @@ else
   CURRENT_BRANCH=$GITHUB_HEAD_REF
 
   if [[ -z $INPUT_BASE_SHA ]]; then
+    git fetch --no-tags -u --progress --deepen=30 origin "$TARGET_BRANCH":"$TARGET_BRANCH"
     PREVIOUS_SHA=$GITHUB_EVENT_PULL_REQUEST_BASE_SHA && exit_status=$? || exit_status=$?
     echo "::debug::Previous SHA: $PREVIOUS_SHA"
   else
+    git fetch --no-tags -u --progress --deepen=30 origin "$INPUT_BASE_SHA" && exit_status=$? || exit_status=$?
     PREVIOUS_SHA=$INPUT_BASE_SHA && exit_status=$? || exit_status=$?
   fi