diff --git a/mutagen/mp4/__init__.py b/mutagen/mp4/__init__.py
index 9e1757ec..c8ab1a76 100644
--- a/mutagen/mp4/__init__.py
+++ b/mutagen/mp4/__init__.py
@@ -820,10 +820,12 @@ def __render_text(self, key, value, flags=AtomDataType.UTF8):
 
         encoded = []
         for v in value:
-            if not isinstance(v, str):
-                raise TypeError("%r not str" % v)
-
-            encoded.append(v.encode("utf-8"))
+            if isinstance(v, bytes):
+                encoded.append(v)
+            elif isinstance(v, str):
+                encoded.append(v.encode("utf-8"))
+            else:
+                raise TypeError("%r neither str or bytes" % v)
 
         return self.__render_data(key, 0, flags, encoded)