diff --git a/tests/fakegithub.py b/tests/fakegithub.py
index 2c226fe4fc..91245b3ce2 100644
--- a/tests/fakegithub.py
+++ b/tests/fakegithub.py
@@ -31,16 +31,6 @@ from zuul.driver.github.githubconnection import utc
 FAKE_BASE_URL = 'https://example.com/api/v3/'
 
 
-class ErrorResponse:
-    status_code = 0
-    message = ''
-
-    def json(self):
-        return {
-            'message': self.message
-        }
-
-
 class FakeUser(object):
     def __init__(self, login):
         self.login = login
@@ -329,11 +319,7 @@ class FakeRepository(object):
 
         if self.fail_not_found > 0:
             self.fail_not_found -= 1
-
-            resp = ErrorResponse()
-            resp.status_code = 404
-            resp.message = 'Not Found'
-
+            resp = FakeResponse(404, 'Not found')
             raise github3.exceptions.NotFoundError(resp)
 
         commit = self._commits.get(sha, None)
@@ -565,8 +551,9 @@ class FakeIssueSearchResult(object):
 
 
 class FakeResponse(object):
-    def __init__(self, data, status_code=200):
+    def __init__(self, data, status_code=200, status_message='OK'):
         self.status_code = status_code
+        self.status_message = status_message
         self.data = data
         self.links = {}
 
@@ -581,8 +568,9 @@ class FakeResponse(object):
 
     def raise_for_status(self):
         if 400 <= self.status_code < 600:
-            raise HTTPError('{} Something went wrong'.format(self.status_code),
-                            response=self)
+            raise HTTPError(
+                '{} {}'.format(self.status_code, self.status_message),
+                response=self)
 
 
 class FakeGithubSession(object):
@@ -667,10 +655,12 @@ class FakeGithubSession(object):
                 raise Exception('Unknown merge failure')
             if conn.merge_not_allowed_count > 0:
                 conn.merge_not_allowed_count -= 1
-                resp = ErrorResponse()
-                resp.status_code = 403
-                resp.message = 'Merge not allowed'
-                raise github3.exceptions.MethodNotAllowed(resp)
+                # GitHub returns 405 Method not allowed with more details in
+                # the body of the response.
+                data = {
+                    'message': 'Merge not allowed because of fake reason',
+                }
+                return FakeResponse(data, 405, 'Method not allowed')
             pr.setMerged(json["commit_message"])
             return FakeResponse({"merged": True}, 200)
 
diff --git a/tests/unit/test_github_driver.py b/tests/unit/test_github_driver.py
index fe79e41918..35c8d92f53 100644
--- a/tests/unit/test_github_driver.py
+++ b/tests/unit/test_github_driver.py
@@ -675,7 +675,8 @@ class TestGithubDriver(ZuulTestCase):
         # Validate that the merge failure comment contains the message github
         # returned
         self.assertEqual(D.comments[0],
-                         'Pull request merge failed: 403 Merge not allowed')
+                         'Pull request merge failed: Merge not allowed '
+                         'because of fake reason')
 
     @simple_layout('layouts/merging-github.yaml', driver='github')
     def test_report_pull_merge_message_reviewed_by(self):
diff --git a/zuul/driver/github/githubconnection.py b/zuul/driver/github/githubconnection.py
index f0452f8fe0..50433ef518 100644
--- a/zuul/driver/github/githubconnection.py
+++ b/zuul/driver/github/githubconnection.py
@@ -1892,6 +1892,14 @@ class GithubConnection(BaseConnection):
             else:
                 result = data["merged"]
         except Exception as e:
+            if hasattr(e, 'response'):
+                response = e.response
+                try:
+                    raise MergeFailure('Pull request merge failed: '
+                                       '%s' % response.json().get('message'))
+                except ValueError:
+                    # There was no json body so use the generic message below.
+                    pass
             raise MergeFailure('Pull request merge failed: %s' % e)
 
         if not result: