Steps to replicate the issue (include links if applicable):

• Go to https://wikifunctions.beta.wmflabs.org/wiki/Z10274
• Click on details

What happens?:
The test fails

What should have happened instead?:
It should pass

The error message is:

Metadata
get last letter python
last letter of Asia is a
Help
Error summary:
[Z507/Error in evaluation]
[Z500/Generic error] Key value not wellformed
Validator error summary:
[Z507/Error in evaluation]
[Z500/Generic error] Key value not wellformed
Expected result: a
Actual result: {"Z1K1":"Z5","Z5K1":"Z507","Z5K2":{"Z1K1":{"Z1K1":"Z7","Z7K1":"Z885","Z885K1":"Z507"},"K1":{"Z1K1":"Z7","Z7K1":{"Z1K1":"Z8","Z8K1":["Z17",{"Z1K1":"Z17","Z17K1":"Z6","Z17K2":"Z10274K1","Z17K3":{"Z1K1":"Z12","Z12K1":["Z11",{"Z1K1":"Z11","Z11K1":"Z1002","Z11K2":"input"},{"Z1K1":"Z11","Z11K1":"Z1430","Z11K2":"Eingabe"}]}}],"Z8K2":"Z6","Z8K3":["Z20"],"Z8K4":["Z14","Z10275"],"Z8K5":"Z10274"},"Z10274K1":"Asia"},"K2":{"Z1K1":"Z5","Z5K1":"Z500","Z5K2":{"Z1K1":{"Z1K1":"Z7","Z7K1":"Z885","Z885K1":"Z500"},"K1":"Key value not wellformed"}}}}

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

If I remove the label on the argument, the test passes. If I add it again, it fails.