diff --git a/src/calibre/db/lazy.py b/src/calibre/db/lazy.py
index 33594767e6..2899f85b27 100644
--- a/src/calibre/db/lazy.py
+++ b/src/calibre/db/lazy.py
@@ -387,7 +387,8 @@ class ProxyMetadata(Metadata):
                                    "Use the explicit methods such as all_field_keys()")
 
     def has_key(self, key):
-        return key in self.all_field_keys()
+        from calibre.ebooks.metadata.book import STANDARD_METADATA_FIELDS
+        return key in STANDARD_METADATA_FIELDS or key in ga(self, '_user_metadata')
 
     def deepcopy(self, **kwargs):
         self._unimplemented_exception('deepcopy', add_txt=False)
diff --git a/src/calibre/ebooks/metadata/book/base.py b/src/calibre/ebooks/metadata/book/base.py
index 1d62c21f99..1e42a10426 100644
--- a/src/calibre/ebooks/metadata/book/base.py
+++ b/src/calibre/ebooks/metadata/book/base.py
@@ -201,7 +201,7 @@ class Metadata:
         return iter(object.__getattribute__(self, '_data'))
 
     def has_key(self, key):
-        return key in object.__getattribute__(self, '_data')
+        return key in STANDARD_METADATA_FIELDS or key in object.__getattribute__(self, '_data')['user_metadata']
 
     def _evaluate_all_composites(self):
         custom_fields = object.__getattribute__(self, '_data')['user_metadata']