diff --git a/src/tld/base.py b/src/tld/base.py
index cdafec9..123f1fb 100644
--- a/src/tld/base.py
+++ b/src/tld/base.py
@@ -1,5 +1,6 @@
 from codecs import open as codecs_open
 from urllib.request import urlopen
+from typing import Optional
 
 from .exceptions import (
     TldIOError,
@@ -17,9 +18,9 @@
 class BaseTLDSourceParser(metaclass=Registry):
     """Base TLD source parser."""
 
-    uid: str = None
-    source_url: str = None
-    local_path: str = None
+    uid: Optional[str] = None
+    source_url: str
+    local_path: str
 
     @classmethod
     def validate(cls):
diff --git a/src/tld/utils.py b/src/tld/utils.py
index 68e4abb..6465eef 100644
--- a/src/tld/utils.py
+++ b/src/tld/utils.py
@@ -200,7 +200,6 @@ def get_tld_names(
         ):
             return _tld_names
 
-        local_file = None
         try:
             # Load the TLD names file
             if isabs(cls.local_path):
@@ -441,6 +440,9 @@ def get_fld(
     if domain_parts is None:
         return None
 
+    # This should be None when domain_parts is None
+    # but mypy isn't quite smart enough to figure that out yet
+    assert non_zero_i is not None
     if non_zero_i < 0:
         # hostname = tld
         return parsed_url.hostname
@@ -497,6 +499,10 @@ def get_tld(
     if domain_parts is None:
         return None
 
+    # This should be None when domain_parts is None
+    # but mypy isn't quite smart enough to figure that out yet
+    assert non_zero_i is not None
+
     if not as_object:
         if non_zero_i < 0:
             # hostname = tld
@@ -507,6 +513,9 @@ def get_tld(
         # hostname = tld
         subdomain = ""
         domain = ""
+        # This is checked in process_url but the type is ambiguous (Optional[str])
+        # so this assertion is just to satisfy mypy
+        assert parsed_url.hostname is not None, "No hostname in URL"
         _tld = parsed_url.hostname
     else:
         subdomain = ".".join(domain_parts[:non_zero_i-1])