diff --git a/highfive/newpr.py b/highfive/newpr.py
index 223570e0..606ce48e 100644
--- a/highfive/newpr.py
+++ b/highfive/newpr.py
@@ -34,8 +34,8 @@
 review_with_reviewer = 'r? @%s\n\n(rust-highfive has picked a reviewer for you, use r? to override)'
 review_without_reviewer = '@%s: no appropriate reviewer found, use r? to override'
 
-reviewer_re = re.compile("\\b[rR]\?[:\- ]*@([a-zA-Z0-9\-]+)")
-submodule_re = re.compile(".*\+Subproject\scommit\s.*", re.DOTALL | re.MULTILINE)
+reviewer_re = re.compile(r"\\b[rR]\?[:\- ]*@([a-zA-Z0-9\-]+)")
+submodule_re = re.compile(r".*\+Subproject\scommit\s.*", re.DOTALL | re.MULTILINE)
 
 rustaceans_api_url = "http://www.ncameron.org/rustaceans/user?username={username}"
 
@@ -105,16 +105,25 @@ def api_req(self, method, url, data=None, media_type=None):
 
     def set_assignee(self, assignee, owner, repo, issue, user, author, to_mention):
         try:
+            assignees = [] if assignee == 'ghost' else [assignee]
+
             self.api_req(
                 "PATCH", issue_url % (owner, repo, issue),
-                {"assignee": assignee}
+                {"assignees": assignees}
             )['body']
         except urllib.error.HTTPError as e:
             if e.code == 201:
                 pass
             else:
                 print(f"failed to assign {assignee} to {owner}/{repo}#{issue}")
-                raise e
+                print(f"error was: {e}")
+                if assignee == 'ghost':
+                    raise Exception('assigned ghost')
+                print("posting error comment")
+                error_msg = f":stop_sign: @{assignee} could not be assigned. " \
+                             "(They may not be a member of **`@rust-lang`**!) " \
+                             "Please assign someone else with `r? @person`."
+                self.post_comment(error_msg, owner, repo, issue)
 
         self.run_commands(to_mention, owner, repo, issue, user)
 
@@ -418,8 +427,8 @@ def new_comment(self):
         # Check the commenter is the submitter of the PR or the previous assignee.
         author = self.payload['issue', 'user', 'login']
         if not (author == commenter or (
-                self.payload['issue', 'assignee'] \
-                and commenter == self.payload['issue', 'assignee', 'login']
+                self.payload['issue', 'assignees'] \
+                and commenter in map(lambda user: user['login'], self.payload['issue', 'assignees'])
         )):
             # Check if commenter is a collaborator.
             if not self.is_collaborator(commenter, owner, repo):
diff --git a/highfive/tests/test_integration_tests.py b/highfive/tests/test_integration_tests.py
index 072b7418..df6d8750 100644
--- a/highfive/tests/test_integration_tests.py
+++ b/highfive/tests/test_integration_tests.py
@@ -88,7 +88,7 @@ def test_new_pr_non_contributor(self):
             (
                 (
                     'PATCH', newpr.issue_url % ('rust-lang', 'rust', '7'),
-                    {'assignee': 'nrc'}
+                    {'assignees': ['nrc']}
                 ),
                 {'body': {}},
             ),
@@ -130,7 +130,7 @@ def test_new_pr_empty_body(self):
             (
                 (
                     'PATCH', newpr.issue_url % ('rust-lang', 'rust', '7'),
-                    {'assignee': 'nrc'}
+                    {'assignees': ['nrc']}
                 ),
                 {'body': {}},
             ),
@@ -171,7 +171,7 @@ def test_new_pr_contributor(self):
             (
                 (
                     'PATCH', newpr.issue_url % ('rust-lang', 'rust', '7'),
-                    {'assignee': 'nrc'}
+                    {'assignees': ['nrc']}
                 ),
                 {'body': {}},
             ),
@@ -215,7 +215,7 @@ def test_new_pr_contributor_with_labels(self):
             (
                 (
                     'PATCH', newpr.issue_url % ('rust-lang', 'rust', '7'),
-                    {'assignee': 'nrc'}
+                    {'assignees': ['nrc']}
                 ),
                 {'body': {}},
             ),
@@ -266,7 +266,7 @@ def test_author_is_commenter(self):
             (
                 (
                     'PATCH', newpr.issue_url % ('rust-lang', 'rust', '1'),
-                    {'assignee': 'davidalber'}
+                    {'assignees': ['davidalber']}
                 ),
                 {'body': {}},
             ),
@@ -293,7 +293,7 @@ def test_author_not_commenter_is_collaborator(self):
             (
                 (
                     'PATCH', newpr.issue_url % ('rust-lang', 'rust', '1'),
-                    {'assignee': 'davidalber'}
+                    {'assignees': ['davidalber']}
                 ),
                 {'body': {}},
             ),
diff --git a/highfive/tests/test_newpr.py b/highfive/tests/test_newpr.py
index cd6a892a..31ca7b4a 100644
--- a/highfive/tests/test_newpr.py
+++ b/highfive/tests/test_newpr.py
@@ -511,7 +511,7 @@ def assert_api_req_call(self, assignee=''):
             'PATCH',
             'https://api.github.com/repos/%s/%s/issues/%s' % (
                 self.owner, self.repo, self.issue
-            ), {"assignee": assignee}
+            ), {"assignees": [assignee]}
         )
 
     def test_api_req_good(self):
@@ -908,7 +908,7 @@ def make_handler(
             'issue': {
                 'state': state,
                 'number': issue_number,
-                'assignee': None,
+                'assignees': [],
                 'user': {
                     'login': author,
                 },
@@ -930,7 +930,7 @@ def make_handler(
         if is_pull_request:
             payload._payload['issue']['pull_request'] = {}
         if assignee is not None:
-            payload._payload['issue']['assignee'] = {'login': assignee}
+            payload._payload['issue']['assignees'] = [{'login': assignee}]
 
         return HighfiveHandlerMock(payload).handler