 KeyError: '21'

[note]: ported from redmine 

Description

No clue what this might be.
Did't use the plugin for several versions of QGIS. Fresh install, no other plugins activated.

KeyError: '21'
Traceback (most recent call last):
File "/home/bernd/.qgis2/python/plugins/EasyCustomLabeling/EasyCustomLabeling.py", line 516, in runLabel
labelFeature[i] = a
KeyError: '21'

2.16 on Linux Mint 17
