diff --git a/lib/util/git/index.spec.ts b/lib/util/git/index.spec.ts
index 87aae87eb3ae46..4574508925a0b9 100644
--- a/lib/util/git/index.spec.ts
+++ b/lib/util/git/index.spec.ts
@@ -136,6 +136,9 @@ describe('platform/git', () => {
     });
   });
   describe('isBranchModified()', () => {
+    it('should return false when branch is not found', async () => {
+      expect(await git.isBranchModified('renovate/not_found')).toBe(false);
+    });
     it('should return true when author matches', async () => {
       expect(await git.isBranchModified('renovate/future_branch')).toBe(false);
       expect(await git.isBranchModified('renovate/future_branch')).toBe(false);
diff --git a/lib/util/git/index.ts b/lib/util/git/index.ts
index 5885488c3a23f0..d43b0b8a3f631a 100644
--- a/lib/util/git/index.ts
+++ b/lib/util/git/index.ts
@@ -401,6 +401,13 @@ export async function isBranchModified(branchName: string): Promise<boolean> {
   if (config.branchIsModified[branchName] !== undefined) {
     return config.branchIsModified[branchName];
   }
+  if (!branchExists(branchName)) {
+    logger.debug(
+      { branchName },
+      'Branch does not exist - cannot check isModified'
+    );
+    return false;
+  }
   // Retrieve the author of the most recent commit
   const lastAuthor = (
     await git.raw([