diff --git a/diff-sha.sh b/diff-sha.sh
index fe2bcd48..153b5414 100644
--- a/diff-sha.sh
+++ b/diff-sha.sh
@@ -52,7 +52,11 @@ if [[ -z $GITHUB_BASE_REF ]]; then
   CURRENT_BRANCH=$TARGET_BRANCH
 
   if [[ -z $INPUT_BASE_SHA ]]; then
-    PREVIOUS_SHA=$(git rev-list --no-merges -n 1 HEAD^1 2>&1) && exit_status=$? || exit_status=$?
+    if [[ $(git rev-list --count HEAD) -gt 1 ]]; then
+      PREVIOUS_SHA=$(git rev-list --no-merges -n 1 HEAD^1 2>&1) && exit_status=$? || exit_status=$?
+    else
+      PREVIOUS_SHA=$CURRENT_SHA
+    fi
   else
     PREVIOUS_SHA=$INPUT_BASE_SHA && exit_status=$? || exit_status=$?
     TARGET_BRANCH=$(git name-rev --name-only "$PREVIOUS_SHA" 2>&1) && exit_status=$? || exit_status=$?