diff --git a/dist/index.js b/dist/index.js
index 71893ad6..b9fb2405 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -3865,29 +3865,30 @@ async function getChangesOnHead() {
     return parseGitDiffOutput(output);
 }
 exports.getChangesOnHead = getChangesOnHead;
-async function getChangesSinceMergeBase(baseRef, ref, initialFetchDepth) {
+async function getChangesSinceMergeBase(base, initialFetchDepth) {
+    const baseRef = `remotes/origin/${base}`;
     async function hasMergeBase() {
-        return (await exec_1.default('git', ['merge-base', baseRef, ref], { ignoreReturnCode: true })).code === 0;
+        return (await exec_1.default('git', ['merge-base', baseRef, exports.HEAD], { ignoreReturnCode: true })).code === 0;
     }
     let noMergeBase = false;
-    core.startGroup(`Searching for merge-base ${baseRef}...${ref}`);
+    core.startGroup(`Searching for merge-base ${baseRef}...${exports.HEAD}`);
     try {
         let init = true;
         let lastCommitCount = await getCommitCount();
         let depth = Math.max(lastCommitCount * 2, initialFetchDepth);
         while (!(await hasMergeBase())) {
             if (init) {
-                await exec_1.default('git', ['fetch', `--depth=${depth}`, 'origin', `${baseRef}:${baseRef}`, `${ref}`]);
+                await exec_1.default('git', ['fetch', `--depth=${depth}`, 'origin', base, exports.HEAD]);
                 init = false;
             }
             else {
-                await exec_1.default('git', ['fetch', `--deepen=${depth}`, 'origin', baseRef, ref]);
+                await exec_1.default('git', ['fetch', `--deepen=${depth}`, 'origin', base, exports.HEAD]);
             }
             const commitCount = await getCommitCount();
             if (commitCount === lastCommitCount) {
                 core.info('No more commits were fetched');
                 core.info('Last attempt will be to fetch full history');
-                await exec_1.default('git', ['fetch', '--unshallow']);
+                await exec_1.default('git', ['fetch']);
                 if (!(await hasMergeBase())) {
                     noMergeBase = true;
                 }
@@ -3905,11 +3906,11 @@ async function getChangesSinceMergeBase(baseRef, ref, initialFetchDepth) {
         return await listAllFilesAsAdded();
     }
     // Get changes introduced on HEAD compared to ref
-    core.startGroup(`Change detection ${baseRef}...${ref}`);
+    core.startGroup(`Change detection ${baseRef}...${exports.HEAD}`);
     let output = '';
     try {
         // Three dots '...' change detection - finds merge-base and compares against it
-        output = (await exec_1.default('git', ['diff', '--no-renames', '--name-status', '-z', `${baseRef}...${ref}`])).stdout;
+        output = (await exec_1.default('git', ['diff', '--no-renames', '--name-status', '-z', `${baseRef}...${exports.HEAD}`])).stdout;
     }
     finally {
         fixStdOutNullTermination();
@@ -4735,7 +4736,7 @@ async function getChangedFilesFromGit(base, initialFetchDepth) {
         if (baseSha === git.NULL_SHA) {
             if (defaultRef && baseRef !== defaultRef) {
                 core.info(`First push of a branch detected - changes will be detected against the default branch ${defaultRef}`);
-                return await git.getChangesSinceMergeBase(defaultRef, ref, initialFetchDepth);
+                return await git.getChangesSinceMergeBase(defaultRef, initialFetchDepth);
             }
             else {
                 core.info('Initial push detected - all files will be listed as added');
@@ -4747,7 +4748,7 @@ async function getChangedFilesFromGit(base, initialFetchDepth) {
     }
     // Changes introduced by current branch against the base branch
     core.info(`Changes will be detected against the branch ${baseRef}`);
-    return await git.getChangesSinceMergeBase(baseRef, ref, initialFetchDepth);
+    return await git.getChangesSinceMergeBase(baseRef, initialFetchDepth);
 }
 // Uses github REST api to get list of files changed in PR
 async function getChangedFilesFromApi(token, pullRequest) {
diff --git a/src/git.ts b/src/git.ts
index a55faa6b..b31dedf2 100644
--- a/src/git.ts
+++ b/src/git.ts
@@ -54,33 +54,31 @@ export async function getChangesOnHead(): Promise<File[]> {
   return parseGitDiffOutput(output)
 }
 
-export async function getChangesSinceMergeBase(
-  baseRef: string,
-  ref: string,
-  initialFetchDepth: number
-): Promise<File[]> {
+export async function getChangesSinceMergeBase(base: string, initialFetchDepth: number): Promise<File[]> {
+  const baseRef = `remotes/origin/${base}`
+
   async function hasMergeBase(): Promise<boolean> {
-    return (await exec('git', ['merge-base', baseRef, ref], {ignoreReturnCode: true})).code === 0
+    return (await exec('git', ['merge-base', baseRef, HEAD], {ignoreReturnCode: true})).code === 0
   }
 
   let noMergeBase = false
-  core.startGroup(`Searching for merge-base ${baseRef}...${ref}`)
+  core.startGroup(`Searching for merge-base ${baseRef}...${HEAD}`)
   try {
     let init = true
     let lastCommitCount = await getCommitCount()
     let depth = Math.max(lastCommitCount * 2, initialFetchDepth)
     while (!(await hasMergeBase())) {
       if (init) {
-        await exec('git', ['fetch', `--depth=${depth}`, 'origin', `${baseRef}:${baseRef}`, `${ref}`])
+        await exec('git', ['fetch', `--depth=${depth}`, 'origin', base, HEAD])
         init = false
       } else {
-        await exec('git', ['fetch', `--deepen=${depth}`, 'origin', baseRef, ref])
+        await exec('git', ['fetch', `--deepen=${depth}`, 'origin', base, HEAD])
       }
       const commitCount = await getCommitCount()
       if (commitCount === lastCommitCount) {
         core.info('No more commits were fetched')
         core.info('Last attempt will be to fetch full history')
-        await exec('git', ['fetch', '--unshallow'])
+        await exec('git', ['fetch'])
         if (!(await hasMergeBase())) {
           noMergeBase = true
         }
@@ -99,11 +97,11 @@ export async function getChangesSinceMergeBase(
   }
 
   // Get changes introduced on HEAD compared to ref
-  core.startGroup(`Change detection ${baseRef}...${ref}`)
+  core.startGroup(`Change detection ${baseRef}...${HEAD}`)
   let output = ''
   try {
     // Three dots '...' change detection - finds merge-base and compares against it
-    output = (await exec('git', ['diff', '--no-renames', '--name-status', '-z', `${baseRef}...${ref}`])).stdout
+    output = (await exec('git', ['diff', '--no-renames', '--name-status', '-z', `${baseRef}...${HEAD}`])).stdout
   } finally {
     fixStdOutNullTermination()
     core.endGroup()
diff --git a/src/main.ts b/src/main.ts
index e3d6ebc4..d9077963 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -109,7 +109,7 @@ async function getChangedFilesFromGit(base: string, initialFetchDepth: number):
     if (baseSha === git.NULL_SHA) {
       if (defaultRef && baseRef !== defaultRef) {
         core.info(`First push of a branch detected - changes will be detected against the default branch ${defaultRef}`)
-        return await git.getChangesSinceMergeBase(defaultRef, ref, initialFetchDepth)
+        return await git.getChangesSinceMergeBase(defaultRef, initialFetchDepth)
       } else {
         core.info('Initial push detected - all files will be listed as added')
         return await git.listAllFilesAsAdded()
@@ -122,7 +122,7 @@ async function getChangedFilesFromGit(base: string, initialFetchDepth: number):
 
   // Changes introduced by current branch against the base branch
   core.info(`Changes will be detected against the branch ${baseRef}`)
-  return await git.getChangesSinceMergeBase(baseRef, ref, initialFetchDepth)
+  return await git.getChangesSinceMergeBase(baseRef, initialFetchDepth)
 }
 
 // Uses github REST api to get list of files changed in PR