diff --git a/GitHubClient.py b/GitHubClient.py
index f38db91..15db432 100644
--- a/GitHubClient.py
+++ b/GitHubClient.py
@@ -17,7 +17,7 @@ class GitHubClient(object):
         self.before = os.getenv('INPUT_BEFORE')
         self.sha = os.getenv('INPUT_SHA')
         self.commits = json.loads(os.getenv('INPUT_COMMITS')) or []
-        self.diff_url = os.getenv('INPUT_DIFF_URL')
+        self.__init_diff_url__()
         self.token = os.getenv('INPUT_TOKEN')
         self.issues_url = f'{self.repos_url}{self.repo}/issues'
         self.milestones_url = f'{self.repos_url}{self.repo}/milestones'
@@ -45,6 +45,20 @@ class GitHubClient(object):
         # Populate milestones so we can perform a lookup if one is specified.
         self._get_milestones()
 
+    def __init_diff_url__(self):
+        manual_commit_ref = os.getenv('MANUAL_COMMIT_REF')
+        manual_base_ref = os.getenv('MANUAL_BASE_REF')
+        if manual_commit_ref:
+            self.sha = manual_commit_ref
+        if manual_commit_ref and manual_base_ref:
+            print(f'Manually comparing {manual_base_ref}...{manual_commit_ref}')
+            self.diff_url = f'{self.repos_url}{self.repo}/compare/{manual_base_ref}...{manual_commit_ref}'
+        elif manual_commit_ref:
+            print(f'Manual checking {manual_commit_ref}')
+            self.diff_url = f'{self.repos_url}{self.repo}/commits/{manual_commit_ref}'
+        else:
+            self.diff_url = os.getenv('INPUT_DIFF_URL')
+
     def get_last_diff(self):
         """Get the last diff."""
         if self.diff_url:
@@ -56,10 +70,12 @@ class GitHubClient(object):
         elif len(self.commits) == 1:
             # There is only one commit.
             diff_url = f'{self.repos_url}{self.repo}/commits/{self.sha}'
-        else:
+        elif len(self.commits) > 1:
             # There are several commits: compare with the oldest one.
             oldest = sorted(self.commits, key=self._get_timestamp)[0]['id']
             diff_url = f'{self.repos_url}{self.repo}/compare/{oldest}...{self.sha}'
+        else:
+            return None
 
         diff_headers = {
             'Accept': 'application/vnd.github.v3.diff',
diff --git a/LocalClient.py b/LocalClient.py
index 9d0b606..b15a495 100644
--- a/LocalClient.py
+++ b/LocalClient.py
@@ -1,13 +1,36 @@
 import subprocess
+import os
 
 class LocalClient(object):
     def __init__(self):
         self.diff_url = None
         self.commits = ['placeholder'] # content doesn't matter, just length
         self.insert_issue_urls = False
+        self.__set_diff_refs__()
+
+    def __set_diff_refs__(self):
+        # set the target of the comparison to user-specified value, if
+        # provided, falling back to HEAD
+        manual_commit_ref = os.getenv('MANUAL_COMMIT_REF')
+        if manual_commit_ref:
+            self.sha = manual_commit_ref
+        else:
+            self.sha = subprocess.run(['git', 'rev-parse', 'HEAD'], stdout=subprocess.PIPE).stdout.decode('utf-8').strip()
+        # set the soruce of the comparison to user-specified value, if
+        # provided, falling back to commit immediately before the target
+        manual_base_ref = os.getenv('MANUAL_BASE_REF')
+        if manual_base_ref:
+            self.base_ref = manual_base_ref
+        else:
+            self.base_ref = subprocess.run(['git', 'rev-parse', f'{self.sha}^'], stdout=subprocess.PIPE).stdout.decode('utf-8').strip()
+        # print feedback to the user
+        if manual_commit_ref and manual_base_ref:
+            print(f'Manually comparing {manual_base_ref}...{manual_commit_ref}')
+        elif manual_commit_ref:
+            print(f'Manual checking {manual_commit_ref}')
 
     def get_last_diff(self):
-        return subprocess.run(['git', 'diff', 'HEAD^..HEAD'], stdout=subprocess.PIPE).stdout.decode('utf-8')
+        return subprocess.run(['git', 'diff', f'{self.base_ref}..{self.sha}'], stdout=subprocess.PIPE).stdout.decode('latin-1')
 
     def create_issue(self, issue):
         return [201, None]
diff --git a/main.py b/main.py
index a16f753..63552d0 100644
--- a/main.py
+++ b/main.py
@@ -25,23 +25,12 @@ if __name__ == "__main__":
     # if needed, fall back to using a local client for testing
     client = client or LocalClient()
 
-    # Check to see if the workflow has been run manually.
-    # If so, adjust the client SHA and diff URL to use the manually supplied inputs.
-    manual_commit_ref = os.getenv('MANUAL_COMMIT_REF')
-    manual_base_ref = os.getenv('MANUAL_BASE_REF')
-    if manual_commit_ref:
-        client.sha = manual_commit_ref
-    if manual_commit_ref and manual_base_ref:
-        print(f'Manually comparing {manual_base_ref}...{manual_commit_ref}')
-        client.diff_url = f'{client.repos_url}{client.repo}/compare/{manual_base_ref}...{manual_commit_ref}'
-    elif manual_commit_ref:
-        print(f'Manual checking {manual_commit_ref}')
-        client.diff_url = f'{client.repos_url}{client.repo}/commits/{manual_commit_ref}'
-    if client.diff_url or len(client.commits) != 0:
-        # Get the diff from the last pushed commit.
-        last_diff = StringIO(client.get_last_diff())
+    # Get the diff from the last pushed commit.
+    last_diff = client.get_last_diff()
+
+    if last_diff:
         # Parse the diff for TODOs and create an Issue object for each.
-        raw_issues = TodoParser().parse(last_diff)
+        raw_issues = TodoParser().parse(StringIO(last_diff))
         # This is a simple, non-perfect check to filter out any TODOs that have just been moved.
         # It looks for items that appear in the diff as both an addition and deletion.
         # It is based on the assumption that TODOs will not have identical titles in identical files.