diff --git a/tools/linter_lib/custom_check.py b/tools/linter_lib/custom_check.py
index 24cf35d3a3..1accba1c73 100644
--- a/tools/linter_lib/custom_check.py
+++ b/tools/linter_lib/custom_check.py
@@ -494,6 +494,17 @@ def build_custom_checkers(by_lang):
          'bad_lines': ['desc = models.CharField(null=True)  # type: Text',
                        'stream = models.ForeignKey(Stream, null=True, on_delete=CASCADE)  # type: Stream'],
          },
+        {'pattern': ' = models[.](?!NullBoolean).*\)  # type: Optional',  # Optional tag, except NullBoolean(Field)
+         'exclude_pattern': 'null=True',
+         'include_only': {"zerver/models.py"},
+         'description': 'Model variable annotated with Optional but variable does not have null=true.',
+         'good_lines': ['desc = models.TextField(null=True)  # type: Optional[Text]',
+                        'stream = models.ForeignKey(Stream, null=True, on_delete=CASCADE)  # type: Optional[Stream]',
+                        'desc = models.TextField()  # type: Text',
+                        'stream = models.ForeignKey(Stream, on_delete=CASCADE)  # type: Stream'],
+         'bad_lines': ['desc = models.TextField()  # type: Optional[Text]',
+                       'stream = models.ForeignKey(Stream, on_delete=CASCADE)  # type: Optional[Stream]'],
+         },
     ]) + whitespace_rules + comma_whitespace_rule
     bash_rules = cast(RuleList, [
         {'pattern': '#!.*sh [-xe]',
diff --git a/zerver/models.py b/zerver/models.py
index a228c48ec4..8e811f29a8 100644
--- a/zerver/models.py
+++ b/zerver/models.py
@@ -870,7 +870,7 @@ class PreregistrationUser(models.Model):
 
     realm = models.ForeignKey(Realm, null=True, on_delete=CASCADE)  # type: Optional[Realm]
 
-    invited_as_admin = models.BooleanField(default=False)  # type: Optional[bool]
+    invited_as_admin = models.BooleanField(default=False)  # type: bool
 
 class MultiuseInvite(models.Model):
     referred_by = models.ForeignKey(UserProfile, on_delete=CASCADE)  # Optional[UserProfile]