Normally, I would think it would be easy as well except that this is not coming from two different models. I modeled a part, started a new part, added an Inheritance feature, selected the previous part as the source and then added some offsets for stock. No assembly was used and this has never been a problem in the past. Additionally, I cannot even select the displayed axes for a dimension as the first reference - it pre-highlights and then when I select it, it just de-selects immediately.
As an aside, PTC deserves to be slammed for the "Knowledge Base" as it is functionally useless, especially to search. All the content may be there, but if I can't find it - why bother building it? That I cannot get any information from an actual, quoted error message is a disgrace.