diff --git a/src/calibre/utils/serialize.py b/src/calibre/utils/serialize.py
index 4073e3c537..d900fc2424 100644
--- a/src/calibre/utils/serialize.py
+++ b/src/calibre/utils/serialize.py
@@ -8,6 +8,8 @@ import json, base64
 from functools import partial
 from datetime import datetime
 
+import msgpack
+
 from calibre.utils.iso8601 import parse_iso8601
 
 MSGPACK_MIME = 'application/x-msgpack'
@@ -23,16 +25,18 @@ def encoder(obj, for_json=False):
         return encoded(0, unicode(obj.isoformat()))
     if isinstance(obj, (set, frozenset)):
         return encoded(1, tuple(obj))
-    if obj.__class__.__name__ == 'Metadata':
+    if hasattr(obj, '__calibre_serializable__'):
         from calibre.ebooks.metadata.book.base import Metadata
+        from calibre.library.field_metadata import FieldMetadata, fm_as_dict
         if isinstance(obj, Metadata):
             from calibre.ebooks.metadata.book.serialize import metadata_as_dict
             return encoded(2, metadata_as_dict(obj, encode_cover_data=for_json))
+        elif isinstance(obj, FieldMetadata):
+            return encoded(3, fm_as_dict(obj))
     raise TypeError('Cannot serialize objects of type {}'.format(type(obj)))
 
 
 def msgpack_dumps(data):
-    import msgpack
     return msgpack.packb(data, use_bin_type=True, default=encoder)
 
 
@@ -55,10 +59,16 @@ def decode_metadata(x, for_json):
     return obj
 
 
+def decode_field_metadata(x, for_json):
+    from calibre.library.field_metadata import fm_from_dict
+    return fm_from_dict(x)
+
+
 decoders = (
     lambda x, fj: parse_iso8601(x, assume_utc=True),
     lambda x, fj: set(x),
     decode_metadata,
+    decode_field_metadata,
 )
 
 
@@ -70,7 +80,6 @@ def decoder(obj, for_json=False):
 
 
 def msgpack_loads(data):
-    import msgpack
     return msgpack.unpackb(data, encoding='utf-8', object_hook=decoder)