diff --git a/offlineimap/folder/UIDMaps.py b/offlineimap/folder/UIDMaps.py
index 1a6283c..ac0a9b4 100644
--- a/offlineimap/folder/UIDMaps.py
+++ b/offlineimap/folder/UIDMaps.py
@@ -191,7 +191,8 @@ class MappedIMAPFolder(IMAPFolder):
 
         newluid = self._mb.savemessage(-1, content, flags, rtime)
         if newluid < 1:
-            raise ValueError("Backend could not find uid for message")
+            raise ValueError("Backend could not find uid for message, returned "
+                             "%s" % newluid)
         self.maplock.acquire()
         try:
             self.diskl2r[newluid] = uid