--- a/mailnews/imap/src/nsImapMailFolder.cpp
+++ b/mailnews/imap/src/nsImapMailFolder.cpp
@@ -3722,16 +3722,19 @@ void nsImapMailFolder::FindKeysToAdd(con
   PRBool showDeletedMessages = ShowDeletedMessages();
   int dbIndex=0; // current index into existingKeys
   PRInt32 existTotal, numberOfKnownKeys;
   PRInt32 messageIndex;
 
   numNewUnread = 0;
   existTotal = numberOfKnownKeys = existingKeys.Length();
   flagState->GetNumberOfMessages(&messageIndex);
+  PRBool partialUIDFetch;
+  flagState->GetPartialUIDFetch(&partialUIDFetch);
+
   for (PRInt32 flagIndex=0; flagIndex < messageIndex; flagIndex++)
   {
     PRUint32 uidOfMessage;
     flagState->GetUidOfMessage(flagIndex, &uidOfMessage);
     while ( (flagIndex < numberOfKnownKeys) && (dbIndex < existTotal) &&
         existingKeys[dbIndex] < uidOfMessage)
       dbIndex++;
 
@@ -3744,19 +3747,21 @@ void nsImapMailFolder::FindKeysToAdd(con
       imapMessageFlagsType flags;
       flagState->GetMessageFlags(flagIndex, &flags);
       NS_ASSERTION(uidOfMessage != nsMsgKey_None, "got invalid msg key");
       if (uidOfMessage && uidOfMessage != nsMsgKey_None && (showDeletedMessages || ! (flags & kImapMsgDeletedFlag)))
       {
         if (mDatabase)
         {
           PRBool dbContainsKey;
-          if (NS_SUCCEEDED(mDatabase->ContainsKey(uidOfMessage, &dbContainsKey)) && dbContainsKey)
+          if (NS_SUCCEEDED(mDatabase->ContainsKey(uidOfMessage, &dbContainsKey)) &&
+              dbContainsKey)
           {
-            NS_ASSERTION(PR_FALSE, "db has key - flagState messed up?");
+            // this is expected in the partial uid fetch case.
+            NS_ASSERTION(!partialUIDFetch, "db has key - flagState messed up?");
             continue;
           }
         }
         keysToFetch.AppendElement(uidOfMessage);
         if (! (flags & kImapMsgSeenFlag))
           numNewUnread++;
       }
     }