diff --git a/index.js b/index.js
index 3f80f59..b9ea0ca 100644
--- a/index.js
+++ b/index.js
@@ -29,9 +29,9 @@
       "Missing required input when assuming a Role."
   );
 
-  const {GITHUB_REPOSITORY, GITHUB_WORKFLOW, GITHUB_ACTION, GITHUB_ACTOR, GITHUB_REF, GITHUB_SHA} = process.env;
+  const {GITHUB_REPOSITORY, GITHUB_WORKFLOW, GITHUB_ACTION, GITHUB_ACTOR, GITHUB_SHA} = process.env;
   assert(
-      [GITHUB_REPOSITORY, GITHUB_WORKFLOW, GITHUB_ACTION, GITHUB_ACTOR, GITHUB_REF, GITHUB_SHA].every(isDefined),
+      [GITHUB_REPOSITORY, GITHUB_WORKFLOW, GITHUB_ACTION, GITHUB_ACTOR, GITHUB_SHA].every(isDefined),
       'Missing required environment value. Are you running in GitHub Actions?'
   );
 
@@ -48,10 +48,13 @@
     {Key: 'Workflow', Value: sanitizeGithubWorkflowName(GITHUB_WORKFLOW)},
     {Key: 'Action', Value: GITHUB_ACTION},
     {Key: 'Actor', Value: sanitizeGithubActor(GITHUB_ACTOR)},
-    {Key: 'Branch', Value: GITHUB_REF},
     {Key: 'Commit', Value: GITHUB_SHA},
   ];
 
+  if (isDefined(process.env.GITHUB_REF)) {
+    tagArray.push({Key: 'Branch', Value: process.env.GITHUB_REF});
+  }
+
   const roleSessionTags = roleSkipSessionTagging ? undefined : tagArray;
 
   const assumeRoleRequest = {
diff --git a/index.test.js b/index.test.js
index 4e26449..8fa322a 100644
--- a/index.test.js
+++ b/index.test.js
@@ -22,8 +22,8 @@
     GITHUB_WORKFLOW: 'MY-WORKFLOW-ID',
     GITHUB_ACTION: 'MY-ACTION-NAME',
     GITHUB_ACTOR: 'MY-USERNAME[bot]',
-    GITHUB_REF: 'MY-BRANCH',
     GITHUB_SHA: 'MY-COMMIT-ID',
+    GITHUB_REF: 'MY-BRANCH',
 };
 const GITHUB_ACTOR_SANITIZED = 'MY-USERNAME_bot_'
 
@@ -142,6 +142,26 @@
         expect(core.setSecret).toHaveBeenCalledWith(FAKE_ACCOUNT_ID);
     });
 
+    test('action fails when github env vars are not set', async () => {
+        process.env.SHOW_STACK_TRACE = 'false';
+        core.getInput = jest
+            .fn()
+            .mockImplementation(mockGetInput(ASSUME_ROLE_INPUTS));
+        delete process.env.GITHUB_SHA;
+
+        await run();
+        expect(core.setFailed).toHaveBeenCalledWith('Missing required environment value. Are you running in GitHub Actions?');
+    });
+
+    test('action does not require GITHUB_REF env var', async () => {
+        core.getInput = jest
+            .fn()
+            .mockImplementation(mockGetInput(ASSUME_ROLE_INPUTS));
+        delete process.env.GITHUB_REF;
+
+        await run();
+    });
+
     test('hosted runners can pull creds from a self-hosted environment', async () => {
         const mockInputs = {'aws-region': FAKE_REGION};
         core.getInput = jest
@@ -415,8 +435,8 @@
                 {Key: 'Workflow', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_WORKFLOW},
                 {Key: 'Action', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_ACTION},
                 {Key: 'Actor', Value: GITHUB_ACTOR_SANITIZED},
-                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
                 {Key: 'Commit', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_SHA},
+                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
             ]
         })
     });
@@ -437,8 +457,8 @@
                 {Key: 'Workflow', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_WORKFLOW},
                 {Key: 'Action', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_ACTION},
                 {Key: 'Actor', Value: GITHUB_ACTOR_SANITIZED},
-                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
                 {Key: 'Commit', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_SHA},
+                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
             ]
         })
     });
@@ -459,8 +479,8 @@
                 {Key: 'Workflow', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_WORKFLOW},
                 {Key: 'Action', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_ACTION},
                 {Key: 'Actor', Value: GITHUB_ACTOR_SANITIZED},
-                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
                 {Key: 'Commit', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_SHA},
+                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
             ]
         })
     });
@@ -481,8 +501,8 @@
                 {Key: 'Workflow', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_WORKFLOW},
                 {Key: 'Action', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_ACTION},
                 {Key: 'Actor', Value: GITHUB_ACTOR_SANITIZED},
-                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
                 {Key: 'Commit', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_SHA},
+                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
             ]
         })
     });
@@ -503,8 +523,8 @@
                 {Key: 'Workflow', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_WORKFLOW},
                 {Key: 'Action', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_ACTION},
                 {Key: 'Actor', Value: GITHUB_ACTOR_SANITIZED},
-                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
                 {Key: 'Commit', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_SHA},
+                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
             ],
             ExternalId: 'abcdef'
         })
@@ -530,8 +550,8 @@
                 {Key: 'Workflow', Value: sanitizedWorkflowName},
                 {Key: 'Action', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_ACTION},
                 {Key: 'Actor', Value: GITHUB_ACTOR_SANITIZED},
-                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
                 {Key: 'Commit', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_SHA},
+                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
             ]
         })
     });
@@ -566,8 +586,8 @@
                 {Key: 'Workflow', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_WORKFLOW},
                 {Key: 'Action', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_ACTION},
                 {Key: 'Actor', Value: GITHUB_ACTOR_SANITIZED},
-                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
                 {Key: 'Commit', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_SHA},
+                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
             ]
         })
     });
@@ -588,8 +608,8 @@
                 {Key: 'Workflow', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_WORKFLOW},
                 {Key: 'Action', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_ACTION},
                 {Key: 'Actor', Value: GITHUB_ACTOR_SANITIZED},
-                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
                 {Key: 'Commit', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_SHA},
+                {Key: 'Branch', Value: ENVIRONMENT_VARIABLE_OVERRIDES.GITHUB_REF},
             ]
         })
     });