diff --git a/zerver/lib/export.py b/zerver/lib/export.py
index 84a2b5e62c..0e41154c79 100644
--- a/zerver/lib/export.py
+++ b/zerver/lib/export.py
@@ -28,7 +28,7 @@ from zerver.models import UserProfile, Realm, Client, Huddle, Stream, \
 from zerver.lib.parallel import run_parallel
 from zerver.lib.utils import mkdir_p
 from six.moves import range
-from typing import Any, Callable, Dict, List, Set, Tuple
+from typing import Any, Callable, Dict, List, Optional, Set, Tuple
 
 # Custom mypy types follow:
 Record = Dict[str, Any]
@@ -229,30 +229,34 @@ class Config(object):
         self.source_filter = source_filter
         self.children = []  # type: List[Config]
 
-        if normal_parent:
-            self.parent = normal_parent
+        if normal_parent is not None:
+            self.parent = normal_parent  # type: Optional[Config]
         else:
             self.parent = None
 
-        if virtual_parent and normal_parent:
-            raise Exception('''
+        if virtual_parent is not None and normal_parent is not None:
+            raise ValueError('''
                 If you specify a normal_parent, please
                 do not create a virtual_parent.
                 ''')
 
-        if normal_parent:
+        if normal_parent is not None:
             normal_parent.children.append(self)
-        elif virtual_parent:
+        elif virtual_parent is not None:
             virtual_parent.children.append(self)
-        elif not is_seeded:
-            raise Exception('''
+        elif is_seeded is None:
+            raise ValueError('''
                 You must specify a parent if you are
                 not using is_seeded.
                 ''')
 
-        if self.id_source:
+        if self.id_source is not None:
+            if self.virtual_parent is None:
+                raise ValueError('''
+                    You must specify a virtual_parent if you are
+                    using id_source.''')
             if self.id_source[0] != self.virtual_parent.table:
-                raise Exception('''
+                raise ValueError('''
                     Configuration error.  To populate %s, you
                     want data from %s, but that differs from
                     the table name of your virtual parent (%s),
@@ -277,6 +281,10 @@ def export_from_config(response, config, seed_object=None, context=None):
     if table:
         exported_tables = [table]
     else:
+        if config.custom_tables is None:
+            raise ValueError('''
+                You must specify config.custom_tables if you
+                are not specifying config.table''')
         exported_tables = config.custom_tables
 
     for t in exported_tables:
@@ -306,9 +314,11 @@ def export_from_config(response, config, seed_object=None, context=None):
             data += response[t]
             del response[t]
             logging.info('Deleted temporary %s' % (t,))
+        assert table is not None
         response[table] = data
 
     elif config.use_all:
+        assert model is not None
         query = model.objects.all()
         rows = list(query)
 
@@ -318,10 +328,14 @@ def export_from_config(response, config, seed_object=None, context=None):
         # now we just need to get all the articles
         # contained by the blogs.
         model = config.model
+        assert parent is not None
+        assert parent.table is not None
+        assert config.parent_key is not None
         parent_ids = [r['id'] for r in response[parent.table]]
-        filter_parms = {config.parent_key: parent_ids}
-        if config.filter_args:
+        filter_parms = {config.parent_key: parent_ids}  # type: Dict[str, Any]
+        if config.filter_args is not None:
             filter_parms.update(config.filter_args)
+        assert model is not None
         query = model.objects.filter(**filter_parms)
         rows = list(query)
 
@@ -330,6 +344,7 @@ def export_from_config(response, config, seed_object=None, context=None):
         # need to look at the current response to get all the
         # blog ids from the Article rows we fetched previously.
         model = config.model
+        assert model is not None
         # This will be a tuple of the form ('zerver_article', 'blog').
         (child_table, field) = config.id_source
         child_rows = response[child_table]
@@ -344,6 +359,7 @@ def export_from_config(response, config, seed_object=None, context=None):
 
     # Post-process rows (which won't apply to custom fetches/concats)
     if rows is not None:
+        assert table is not None  # Hint for mypy
         response[table] = make_raw(rows, exclude=config.exclude)
         if table in DATE_FIELDS:
             floatify_datetime_fields(response, table)
@@ -625,6 +641,8 @@ def fetch_huddle_objects(response, config, context):
     # type: (TableData, Config, Context) -> None
 
     realm = context['realm']
+    assert config.parent is not None
+    assert config.parent.table is not None
     user_profile_ids = set(r['id'] for r in response[config.parent.table])
 
     # First we get all huddles involving someone in the realm.
@@ -1415,7 +1433,7 @@ def do_import_realm(import_dir):
     fix_datetime_fields(data, 'zerver_realm')
     realm = Realm(**data['zerver_realm'][0])
     if realm.notifications_stream_id is not None:
-        notifications_stream_id = int(realm.notifications_stream_id)
+        notifications_stream_id = int(realm.notifications_stream_id)  # type: Optional[int]
     else:
         notifications_stream_id = None
     realm.notifications_stream_id = None