diff --git a/beets/library.py b/beets/library.py
index 748f532a5..f294e6d4f 100644
--- a/beets/library.py
+++ b/beets/library.py
@@ -205,6 +205,8 @@ class MusicalKey(types.String):
         r'bb': 'a#',
     }
 
+    null = None
+
     def parse(self, key):
         key = key.lower()
         for flat, sharp in self.ENHARMONIC.items():