diff --git a/diff-sha.sh b/diff-sha.sh
index f2df0b98..426f12bc 100644
--- a/diff-sha.sh
+++ b/diff-sha.sh
@@ -68,18 +68,20 @@ else
 fi
 
 function deepenShallowCloneToFindCommit() {
-  local ref="$1"
-  local target_branch="$2"
+  local base_ref="$1"
+  local diff="$2"
+  local ref="$3"
+  local target_branch="$4"
   local depth=20
   local max_depth=$INPUT_MAX_FETCH_DEPTH
 
-  while ! git rev-parse --quiet --verify "$ref^{commit}" &>/dev/null; do
-    echo "::debug::Unable to find commit '$ref' in shallow clone. Increasing depth to $((depth * 2))..."
+  while ! git diff "$base_ref$diff$ref" &>/dev/null; do
+    echo "::debug::Unable to find merge-base in shallow clone. Increasing depth to $((depth * 2))..."
 
     depth=$((depth * 2))
 
     if [[ $depth -gt $max_depth ]]; then
-      echo "::error::Unable to find commit '$ref' in shallow clone. Maximum depth of $max_depth reached."
+      echo "::error::Unable to find merge-base in shallow clone. Please increase 'max_fetch_depth' to at least $((depth + 20))."
       exit 1
     fi
 
@@ -136,7 +138,7 @@ if [[ -z $GITHUB_BASE_REF ]]; then
   echo "::debug::Current branch $CURRENT_BRANCH..."
 
   echo "::debug::Fetching previous commit SHA: $PREVIOUS_SHA"
-  deepenShallowCloneToFindCommit "$PREVIOUS_SHA" "$TARGET_BRANCH"
+  deepenShallowCloneToFindCommit "$PREVIOUS_SHA" ".." "$CURRENT_SHA" "$TARGET_BRANCH"
 
   echo "::debug::Verifying the previous commit SHA: $PREVIOUS_SHA"
   git rev-parse --quiet --verify "$PREVIOUS_SHA^{commit}" 1>/dev/null 2>&1 && exit_status=$? || exit_status=$?
@@ -162,7 +164,7 @@ else
   echo "::debug::Current branch: $CURRENT_BRANCH"
 
   echo "::debug::Fetching previous commit SHA: $PREVIOUS_SHA"
-  deepenShallowCloneToFindCommit "$PREVIOUS_SHA" "$TARGET_BRANCH"
+  deepenShallowCloneToFindCommit "$PREVIOUS_SHA" "..." "$CURRENT_SHA"  "$TARGET_BRANCH"
 
   echo "::debug::Verifying the previous commit SHA: $PREVIOUS_SHA"
   git rev-parse --quiet --verify "$PREVIOUS_SHA^{commit}" 1>/dev/null 2>&1 && exit_status=$? || exit_status=$?