diff --git a/buildtrigger/customhandler.py b/buildtrigger/customhandler.py
index 7541995a5..193445ee2 100644
--- a/buildtrigger/customhandler.py
+++ b/buildtrigger/customhandler.py
@@ -16,9 +16,6 @@ from buildtrigger.bitbuckethandler import (BITBUCKET_WEBHOOK_PAYLOAD_SCHEMA as b
 from buildtrigger.githubhandler import (GITHUB_WEBHOOK_PAYLOAD_SCHEMA as gh_schema,
                                         get_transformed_webhook_payload as gh_payload)
 
-from buildtrigger.bitbuckethandler import (BITBUCKET_WEBHOOK_PAYLOAD_SCHEMA as bb_schema,
-                                           get_transformed_webhook_payload as bb_payload)
-
 from buildtrigger.gitlabhandler import (GITLAB_WEBHOOK_PAYLOAD_SCHEMA as gl_schema,
                                         get_transformed_webhook_payload as gl_payload)
 
@@ -162,7 +159,7 @@ class CustomBuildTrigger(BuildTriggerHandler):
   def handle_trigger_request(self, request):
     payload = request.data
     if not payload:
-      raise InvalidPayloadException()
+      raise InvalidPayloadException('Missing expected payload')
 
     logger.debug('Payload %s', payload)
 
@@ -186,7 +183,10 @@ class CustomBuildTrigger(BuildTriggerHandler):
       'git_url': config['build_source'],
     }
 
-    return self.prepare_build(metadata, is_manual=True)
+    try:
+      return self.prepare_build(metadata, is_manual=True)
+    except ValidationError as ve:
+      raise TriggerStartException(ve.message)
 
   def activate(self, standard_webhook_url):
     config = self.config
diff --git a/buildtrigger/test/test_customhandler.py b/buildtrigger/test/test_customhandler.py
new file mode 100644
index 000000000..6d05cb2b9
--- /dev/null
+++ b/buildtrigger/test/test_customhandler.py
@@ -0,0 +1,51 @@
+import pytest
+
+from buildtrigger.customhandler import CustomBuildTrigger
+from buildtrigger.triggerutil import (InvalidPayloadException, SkipRequestException,
+                                      TriggerStartException)
+from endpoints.building import PreparedBuild
+from util.morecollections import AttrDict
+
+@pytest.mark.parametrize('payload, expected_error, expected_message', [
+  ('', InvalidPayloadException, 'Missing expected payload'),
+  ('{}', InvalidPayloadException, "'commit' is a required property"),
+
+  ('{"commit": "foo", "ref": "bar", "default_branch": "baz"}',
+   InvalidPayloadException, "u'foo' does not match '^([A-Fa-f0-9]{7,})$'"),
+
+  ('{"commit": "11d6fbc", "ref": "refs/heads/something", "default_branch": "baz"}', None, None),
+  ('''{
+    "commit": "11d6fbc",
+    "ref": "refs/heads/something",
+    "default_branch": "baz",
+    "commit_info": {
+      "message": "[skip build]",
+      "url": "http://foo.bar",
+      "date": "NOW"
+    }
+  }''', SkipRequestException, ''),
+])
+def test_handle_trigger_request(payload, expected_error, expected_message):
+  trigger = CustomBuildTrigger(None, {'build_source': 'foo'})
+  request = AttrDict(dict(data=payload))
+
+  if expected_error is not None:
+    with pytest.raises(expected_error) as ipe:
+      trigger.handle_trigger_request(request)
+    assert ipe.value.message == expected_message
+  else:
+    assert isinstance(trigger.handle_trigger_request(request), PreparedBuild)
+
+@pytest.mark.parametrize('run_parameters, expected_error, expected_message', [
+  ({}, TriggerStartException, 'missing required parameter'),
+  ({'commit_sha': 'foo'}, TriggerStartException, "'foo' does not match '^([A-Fa-f0-9]{7,})$'"),
+  ({'commit_sha': '11d6fbc'}, None, None),
+])
+def test_manual_start(run_parameters, expected_error, expected_message):
+  trigger = CustomBuildTrigger(None, {'build_source': 'foo'})
+  if expected_error is not None:
+    with pytest.raises(expected_error) as ipe:
+      trigger.manual_start(run_parameters)
+    assert ipe.value.message == expected_message
+  else:
+    assert isinstance(trigger.manual_start(run_parameters), PreparedBuild)