diff --git a/action.yml b/action.yml
index bdac475d..3e35809f 100644
--- a/action.yml
+++ b/action.yml
@@ -77,6 +77,10 @@ inputs:
     description: "Output changed files in JSON format which can be used for matrix jobs"
     required: false
     default: "false"
+  target_branch_fetch_depth:
+    description: "Fetch depth for target branch. NOTE: This can be adjusted to resolve errors with insufficient history. See: #668."
+    required: false
+    default: "20"
 
 outputs:
   added_files:
@@ -184,14 +188,15 @@ runs:
         GITHUB_BASE_REF: ${{ github.base_ref }}
         GITHUB_HEAD_REF: ${{ github.head_ref }}
         GITHUB_WORKSPACE: ${{ github.workspace }}
-        GITHUB_PULL_REQUEST_BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        GITHUB_EVENT_PULL_REQUEST_BASE_SHA: ${{ github.event.pull_request.base.sha }}
         GITHUB_EVENT_BEFORE: ${{ github.event.before }}
-        GITHUB_EVENT_FORCED : ${{ github.event.forced }}
+        GITHUB_EVENT_FORCED: ${{ github.event.forced }}
         # INPUT_<VARIABLE_NAME> is not available in Composite run steps
         # https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#example-specifying-inputs
         INPUT_SHA: ${{ steps.sha.outputs.sha }}
         INPUT_BASE_SHA: ${{ steps.base-sha.outputs.base_sha }}
         INPUT_PATH: ${{ inputs.path }}
+        INPUT_TARGET_BRANCH_FETCH_DEPTH: ${{ inputs.target_branch_fetch_depth }}
     - name: Glob match
       uses: tj-actions/glob@v14
       id: glob
diff --git a/diff-sha.sh b/diff-sha.sh
index 551b5d13..6c610cd8 100644
--- a/diff-sha.sh
+++ b/diff-sha.sh
@@ -62,7 +62,7 @@ if [[ -z $GITHUB_BASE_REF ]]; then
   CURRENT_BRANCH=$TARGET_BRANCH && exit_status=$? || exit_status=$?
 
   if [[ -z $INPUT_BASE_SHA ]]; then
-    git fetch --no-tags -u --progress origin --depth=2 "${TARGET_BRANCH}":"${TARGET_BRANCH}" && exit_status=$? || exit_status=$?
+    git fetch --no-tags -u --progress origin --depth="$INPUT_TARGET_BRANCH_FETCH_DEPTH" "${TARGET_BRANCH}":"${TARGET_BRANCH}" && exit_status=$? || exit_status=$?
     PREVIOUS_SHA=""
 
     if [[ "$GITHUB_EVENT_FORCED" == "false" ]]; then
@@ -105,12 +105,13 @@ else
 
   echo "::debug::GITHUB_BASE_REF: $TARGET_BRANCH..."
 
+  git fetch --no-tags -u --progress origin --depth="$INPUT_TARGET_BRANCH_FETCH_DEPTH" "${TARGET_BRANCH}":"${TARGET_BRANCH}" &&
+  exit_status=$? || exit_status=$?
+
   if [[ -z $INPUT_BASE_SHA ]]; then
-    git fetch --no-tags -u --progress origin --depth=1 "${TARGET_BRANCH}":"${TARGET_BRANCH}" && exit_status=$? || exit_status=$?
-    PREVIOUS_SHA=$GITHUB_PULL_REQUEST_BASE_SHA && exit_status=$? || exit_status=$?
+    PREVIOUS_SHA=$GITHUB_EVENT_PULL_REQUEST_BASE_SHA && exit_status=$? || exit_status=$?
     echo "::debug::Previous SHA: $PREVIOUS_SHA"
   else
-    git fetch --no-tags -u --progress origin --depth=1 "$(git rev-parse --verify "$INPUT_BASE_SHA")" && exit_status=$? || exit_status=$?
     PREVIOUS_SHA=$INPUT_BASE_SHA
     TARGET_BRANCH=$(git name-rev --name-only "$PREVIOUS_SHA" 2>&1) && exit_status=$? || exit_status=$?
     echo "::debug::Previous SHA: $PREVIOUS_SHA"