The general flow I follow when merging tags is 

- Propose a synonym from the individual tag page
- [Approve it from tag synonyms][1]
- Merge the questions from both the tags [using the merge option][2]


When following the same for the above tag, I ran into the above error. I went over to the [tag synonym page][1], swapped the mapping & proceeded with the merge, at which point it went fine. 

Still curious to know about the error


  [1]: https://superuser.com/tags/synonyms?filter=all&tab=newest
  [2]: https://superuser.com/tags/synonyms?filter=merge&tab=newest