diff --git a/lib/extras.py b/lib/extras.py
index b21e223d..c108745c 100644
--- a/lib/extras.py
+++ b/lib/extras.py
@@ -644,7 +644,7 @@ class HstoreAdapter(object):
 
         k = _ext.adapt(self.wrapped.keys())
         k.prepare(self.conn)
-        v = _ext.adapt(self.wrapped.values())
+        v = _ext.adapt([unicode(x) if x is not None else None for x in self.wrapped.values()])
         v.prepare(self.conn)
         return b("hstore(") + k.getquoted() + b(", ") + v.getquoted() + b(")")