diff --git a/git_updater.py b/git_updater.py
index 4c02821..ef320cb 100644
--- a/git_updater.py
+++ b/git_updater.py
@@ -66,8 +66,10 @@
         self._new_ver = updater_utils.get_latest_version(self._old_ver, tags)
 
     def _check_head(self) -> None:
+        branch = git_utils.get_default_branch(self._proj_path,
+                                              self.UPSTREAM_REMOTE_NAME)
         self._new_ver = git_utils.get_sha_for_branch(
-            self._proj_path, self.UPSTREAM_REMOTE_NAME + '/master')
+            self._proj_path, self.UPSTREAM_REMOTE_NAME + '/' + branch)
 
     def update(self) -> None:
         """Updates the package.
diff --git a/git_utils.py b/git_utils.py
index 1825c37..f96a600 100644
--- a/git_utils.py
+++ b/git_utils.py
@@ -99,7 +99,7 @@
     stripped = [line.strip() for line in lines]
     remote_path = remote_name + '/'
     return [
-        line.lstrip(remote_path) for line in stripped
+        line[len(remote_path):] for line in stripped
         if line.startswith(remote_path)
     ]
 
@@ -116,6 +116,18 @@
     return list(set(tags))
 
 
+def get_default_branch(proj_path: Path, remote_name: str) -> str:
+    """Gets the name of the upstream branch to use."""
+    branches_to_try = ['master', 'main']
+    remote_branches = list_remote_branches(proj_path, remote_name)
+    for branch in branches_to_try:
+        if branch in remote_branches:
+            return branch
+    # We couldn't find any of the branches we expected.
+    # Default to 'master', although nothing will work well.
+    return 'master'
+
+
 COMMIT_PATTERN = r'^[a-f0-9]{40}$'
 COMMIT_RE = re.compile(COMMIT_PATTERN)