diff --git a/get-modified-packages/action.yaml b/get-modified-packages/action.yaml
index 8b0ff808..4831af82 100644
--- a/get-modified-packages/action.yaml
+++ b/get-modified-packages/action.yaml
@@ -14,10 +14,14 @@ runs:
         git config --global --add safe.directory "$GITHUB_WORKSPACE"
       shell: bash
 
+    - name: Fetch the base branch with enough history for a common merge-base commit
+      run: |
+        git fetch origin ${{ github.event.pull_request.base.ref }}
+
     - name: Get modified packages
       id: get-modified-packages
       run: |
-        echo "modified-packages=$(${GITHUB_ACTION_PATH}/get-modified-packages.sh ${{ github.event.pull_request.base.sha }})" >> $GITHUB_OUTPUT
+        echo "modified-packages=$(${GITHUB_ACTION_PATH}/get-modified-packages.sh origin/${{ github.event.pull_request.base.ref }})" >> $GITHUB_OUTPUT
       shell: bash
 
     - name: Show result
diff --git a/get-modified-packages/get-modified-packages.sh b/get-modified-packages/get-modified-packages.sh
index 4c3ccf6b..f0d8d266 100755
--- a/get-modified-packages/get-modified-packages.sh
+++ b/get-modified-packages/get-modified-packages.sh
@@ -1,6 +1,6 @@
 #!/bin/bash
-# Search for packages that have been modified from the base sha.
-# Usage: get-modified-packages.sh <base_sha>
+# Search for packages that have been modified from the base ref.
+# Usage: get-modified-packages.sh <base_ref>
 
 set -e
 
@@ -15,11 +15,11 @@ while [ "${1-}" != "" ]; do
     shift
 done
 
-base_sha="${args[0]}"
+base_ref="${args[0]}"
 
 # Check args
-if [ "$base_sha" = "" ]; then
-    echo -e "\e[31mPlease input a valid base_sha as the 1st argument.\e[m"
+if [ "$base_ref" = "" ]; then
+    echo -e "\e[31mPlease input a valid base_ref as the 1st argument.\e[m"
     exit 1
 fi
 
@@ -49,8 +49,8 @@ function find_package_dir() {
     return 1
 }
 
-# Find modified files from base sha
-modified_files=$(git diff --name-only "$base_sha"...HEAD)
+# Find modified files from base ref
+modified_files=$(git diff --name-only "$base_ref"...HEAD)
 
 # Find modified packages
 modified_package_dirs=()