diff --git a/sphinx/locale/__init__.py b/sphinx/locale/__init__.py
index d5ca91488..5c2caea9b 100644
--- a/sphinx/locale/__init__.py
+++ b/sphinx/locale/__init__.py
@@ -65,7 +65,7 @@ class _TranslationProxy(UserString, object):
     # for the encoding result
 
     def encode(self, encoding=None, errors=None):  # type: ignore
-        # type: (unicode, unicode) -> str
+        # type: (unicode, unicode) -> bytes
         if encoding:
             if errors:
                 return self.data.encode(encoding, errors)
diff --git a/sphinx/util/images.py b/sphinx/util/images.py
index 9abe748e4..321768670 100644
--- a/sphinx/util/images.py
+++ b/sphinx/util/images.py
@@ -78,7 +78,7 @@ def guess_mimetype_for_stream(stream, default=None):
 
 
 def guess_mimetype(filename='', content=None, default=None):
-    # type: (unicode, unicode, unicode) -> unicode
+    # type: (unicode, bytes, unicode) -> unicode
     _, ext = path.splitext(filename.lower())
     if ext in mime_suffixes:
         return mime_suffixes[ext]
@@ -123,7 +123,7 @@ def parse_data_uri(uri):
 
 
 def test_svg(h, f):
-    # type: (unicode, IO) -> unicode
+    # type: (bytes, IO) -> unicode
     """An additional imghdr library helper; test the header is SVG's or not."""
     try:
         if '<svg' in h.decode('utf-8').lower():