diff --git a/action.yml b/action.yml
index 65cdf02..3cb5b70 100644
--- a/action.yml
+++ b/action.yml
@@ -15,11 +15,15 @@ inputs:
   BEFORE:
     description: "The SHA of the last pushed commit (automatically set)"
     required: true
-    default: "${{ github.event.before }}"     
+    default: "${{ github.event.before || github.base_ref }}"
   COMMITS:
     description: "An array of commit objects describing the pushed commits"
     required: true
     default: "${{ toJSON(github.event.commits) }}"
+  DIFF_URL:
+    description: "The URL to use to get the diff (automatically set)"
+    required: true
+    default: "${{ github.event.pull_request.diff_url }}"
   SHA:
     description: "The SHA of the latest commit (automatically set)"
     required: true
diff --git a/main.py b/main.py
index 5327578..45f9946 100644
--- a/main.py
+++ b/main.py
@@ -51,6 +51,7 @@ class GitHubClient(object):
         self.before = os.getenv('INPUT_BEFORE')
         self.sha = os.getenv('INPUT_SHA')
         self.commits = json.loads(os.getenv('INPUT_COMMITS'))
+        self.diff_url = os.getenv('INPUT_DIFF_URL')
         self.token = os.getenv('INPUT_TOKEN')
         self.issues_url = f'{self.repos_url}{self.repo}/issues'
         self.issue_headers = {
@@ -67,7 +68,10 @@ class GitHubClient(object):
 
     def get_last_diff(self):
         """Get the last diff."""
-        if self.before != '0000000000000000000000000000000000000000':
+        if self.diff_url:
+            # Diff url was directly passed in config, likely due to this being a PR
+            diff_url = self.diff_url
+        elif self.before != '0000000000000000000000000000000000000000':
             # There is a valid before SHA to compare with, or this is a release being created
             diff_url = f'{self.repos_url}{self.repo}/compare/{self.before}...{self.sha}'
         elif len(self.commits) == 1:
@@ -618,7 +622,7 @@ class TodoParser(object):
 if __name__ == "__main__":
     # Create a basic client for communicating with GitHub, automatically initialised with environment variables.
     client = GitHubClient()
-    if len(client.commits) != 0:
+    if client.diff_url or len(client.commits) != 0:
         # Get the diff from the last pushed commit.
         last_diff = StringIO(client.get_last_diff())
         # Parse the diff for TODOs and create an Issue object for each.