diff --git a/__tests__/context.test.ts b/__tests__/context.test.ts
index 98bfcf6a..f45f886f 100644
--- a/__tests__/context.test.ts
+++ b/__tests__/context.test.ts
@@ -58,17 +58,15 @@ describe('parseGitRef', () => {
   });
   // prettier-ignore
   test.each([
-    ['refs/heads/master', '860c1904a1ce19322e91ac35af1ab07466440c37', false, '860c1904a1ce19322e91ac35af1ab07466440c37'],
-    ['master', '860c1904a1ce19322e91ac35af1ab07466440c37', false, '860c1904a1ce19322e91ac35af1ab07466440c37'],
-    ['refs/pull/15/merge', '860c1904a1ce19322e91ac35af1ab07466440c37', false, 'refs/pull/15/merge'],
-    ['refs/heads/master', '', false, 'refs/heads/master'],
-    ['master', '', false, 'master'],
-    ['refs/tags/v1.0.0', '', false, 'refs/tags/v1.0.0'],
-    ['refs/pull/15/merge', '', false, 'refs/pull/15/merge'],
-    ['refs/pull/15/merge', '', true, 'refs/pull/15/head'],
-  ])('given %p and %p, should return %p', async (ref: string, sha: string, prHeadRef: boolean, expected: string) => {
+    ['refs/heads/master', false, 'refs/heads/master'],
+    ['master', false, 'master'],
+    ['refs/pull/15/merge', false, 'refs/pull/15/merge'],
+    ['refs/tags/v1.0.0', false, 'refs/tags/v1.0.0'],
+    ['refs/pull/15/merge', true, 'refs/pull/15/head'],
+    ['', false, '860c1904a1ce19322e91ac35af1ab07466440c37'],
+  ])('given %p and %p, should return %p', async (ref: string, prHeadRef: boolean, expected: string) => {
     process.env.DOCKER_DEFAULT_GIT_CONTEXT_PR_HEAD_REF = prHeadRef ? 'true' : '';
-    expect(Context.parseGitRef(ref, sha)).toEqual(expected);
+    expect(Context.parseGitRef(ref, '860c1904a1ce19322e91ac35af1ab07466440c37')).toEqual(expected);
   });
 });
 
diff --git a/src/context.ts b/src/context.ts
index 502bb3e9..75771ffb 100644
--- a/src/context.ts
+++ b/src/context.ts
@@ -43,13 +43,14 @@ export class Context {
   }
 
   public static parseGitRef(ref: string, sha: string): string {
-    const setPullRequestHeadRef: boolean = !!(process.env.DOCKER_DEFAULT_GIT_CONTEXT_PR_HEAD_REF && process.env.DOCKER_DEFAULT_GIT_CONTEXT_PR_HEAD_REF === 'true');
-    if (sha && ref && !ref.startsWith('refs/')) {
-      ref = `refs/heads/${ref}`;
+    if (!ref) {
+      if (!sha) {
+        throw new Error('Git reference or commit SHA is required');
+      }
+      return sha;
     }
-    if (sha && !ref.startsWith(`refs/pull/`)) {
-      ref = sha;
-    } else if (ref.startsWith(`refs/pull/`) && setPullRequestHeadRef) {
+    const setPullRequestHeadRef: boolean = !!(process.env.DOCKER_DEFAULT_GIT_CONTEXT_PR_HEAD_REF && process.env.DOCKER_DEFAULT_GIT_CONTEXT_PR_HEAD_REF === 'true');
+    if (ref.startsWith(`refs/pull/`) && setPullRequestHeadRef) {
       ref = ref.replace(/\/merge$/g, '/head');
     }
     return ref;