diff --git a/baldrick/github/github_api.py b/baldrick/github/github_api.py
index 683b4f4..6848a8e 100644
--- a/baldrick/github/github_api.py
+++ b/baldrick/github/github_api.py
@@ -10,7 +10,8 @@
 from ttldict import TTLOrderedDict
 
 from baldrick.config import loads
-from baldrick.github.github_auth import github_request_headers
+from baldrick.github.github_auth import (
+    github_request_headers, repo_to_installation_id)
 
 __all__ = ['GitHubHandler', 'RepoHandler', 'PullRequestHandler']
 
@@ -76,6 +77,9 @@ class GitHubHandler:
     A base class for things that represent things the github app can operate on.
     """
     def __init__(self, repo, installation=None):
+        if installation is None:  # pragma: no cover
+            installation = repo_to_installation_id(repo, raise_error=False)
+
         self.repo = repo
         self.installation = installation
         self._cache = {}
diff --git a/baldrick/github/github_auth.py b/baldrick/github/github_auth.py
index 1472396..da121e2 100644
--- a/baldrick/github/github_auth.py
+++ b/baldrick/github/github_auth.py
@@ -136,15 +136,18 @@ def repo_to_installation_id_mapping():
     return repos
 
 
-def repo_to_installation_id(repository):
+def repo_to_installation_id(repository, raise_error=True):
     """
     Return the installation ID for a repository.
     """
     mapping = repo_to_installation_id_mapping()
     if repository in mapping:
         return mapping[repository]
+    elif raise_error:
+        raise ValueError("Repository not recognized - should be one of:\n\n"
+                         "  - " + "\n  - ".join(mapping))
     else:
-        raise ValueError("Repository not recognized - should be one of:\n\n  - " + "\n  - ".join(mapping))
+        return None
 
 
 def get_app_name():
diff --git a/baldrick/github/tests/test_github_auth.py b/baldrick/github/tests/test_github_auth.py
index 6eb6765..f4a9c32 100644
--- a/baldrick/github/tests/test_github_auth.py
+++ b/baldrick/github/tests/test_github_auth.py
@@ -100,6 +100,7 @@ def test_repo_to_installation_id(app):
             with patch('requests.get', requests_patch):
 
                 assert repo_to_installation_id('test1') == 3331
+                assert repo_to_installation_id('test3', raise_error=False) is None
 
                 with pytest.raises(ValueError) as exc:
                     repo_to_installation_id('test3')
diff --git a/baldrick/plugins/tests/test_github_milestones.py b/baldrick/plugins/tests/test_github_milestones.py
index 275e913..51a51b8 100644
--- a/baldrick/plugins/tests/test_github_milestones.py
+++ b/baldrick/plugins/tests/test_github_milestones.py
@@ -28,6 +28,9 @@ class TestMilestonePlugin:
 
     def setup_method(self, method):
 
+        self.get_installation = patch('baldrick.github.github_auth.repo_to_installation_id_mapping')
+        i = self.get_installation.start()
+        i.return_value = {}
         self.get_file_contents_mock = patch('baldrick.github.github_api.PullRequestHandler.get_file_contents')
         self.get_base_branch_mock = patch('baldrick.github.github_api.PullRequestHandler.base_branch')
         a = self.get_base_branch_mock.start()
@@ -45,6 +48,7 @@ def setup_method(self, method):
         cfg_cache.clear()
 
     def teardown_method(self, method):
+        self.get_installation.stop()
         self.get_file_contents_mock.stop()
         self.milestone_mock.stop()
         self.get_base_branch_mock.stop()
diff --git a/baldrick/plugins/tests/test_github_towncrier_changelog.py b/baldrick/plugins/tests/test_github_towncrier_changelog.py
index e8d5924..7aa210c 100644
--- a/baldrick/plugins/tests/test_github_towncrier_changelog.py
+++ b/baldrick/plugins/tests/test_github_towncrier_changelog.py
@@ -20,6 +20,9 @@ class TestTowncrierPlugin:
 
     def setup_method(self, method):
 
+        self.get_installation = patch('baldrick.github.github_auth.repo_to_installation_id_mapping')
+        i = self.get_installation.start()
+        i.return_value = {}
         self.get_file_contents_mock = patch('baldrick.github.github_api.PullRequestHandler.get_file_contents')
         self.get_base_branch_mock = patch('baldrick.github.github_api.PullRequestHandler.base_branch')
         a = self.get_base_branch_mock.start()
@@ -35,6 +38,7 @@ def setup_method(self, method):
         cfg_cache.clear()
 
     def teardown_method(self, method):
+        self.get_installation.stop()
         self.get_file_contents_mock.stop()
         self.modified_files_mock.stop()
         self.get_base_branch_mock.stop()