Non-existent user defined runtime attribute

I have a simple Mac application that receives the following error when opening a document:

Failed to set (allowsDragging) user defined inspected property on (PKMPDFView): [<PKMPDFView 0x608000161f80> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key allowsDragging.


If I intercept the "set" attempt, I see that it originates from the method -[NSIBUserDefinedRuntimeAttributesConnector establishConnection].


What confuses me is that PKMPDFView is a direct subclass of NSView and has no user defined properties associated with it in the .xib file being loaded. This view also has no outlets defined in the .xib, just a referencing outlet (contentView) from its document. Running find/grep through my development directory finds no instance of "allowsDragging" anywhere. Likewise, using the strings utility reveals no "allowsDragging" in the built app.


(I've tried all the standard project cleaning with no change in symptoms.)


At the moment, I'm tempted to simply implement setValue:forUndefinedKey: and forget it but I'm still curious. Does anyone have information or good guesses about where this would be coming from?