What is NSMapTable count underflow?

Question

Fatal Exception: NSInternalInconsistencyException

[NSMapTable NSMapTable { 
[7]  <TextNode: 0x1499dc810> -> NormalCellModelCache 
[17] <TextNode: 0x14990ad50> -> NormalCellModelCache 
[18] <TextNode: 0x14992c560> -> NormalCellModelCache
[21] <TextNode: 0x14989ce80> -> NormalCellModelCache 
[22] <ReplyNode: 0x1496878f0> -> ReplyCellModelCache } ] count underflow

0   CoreFoundation  
__exceptionPreprocess
5   Foundation  
-[NSConcreteMapTable setObject:forKey:]
...

This is a weak and strong NSMapTable. I store the tableViewCell cache in it using the model object as a key. What does "count underflow" mean here?