UE4 crashing when attempting to modify "Target Arm Length" setting on Spring arm component

Basically I have 2 functions named “ZoomCamera” and “RetractCamera” and I have binded the “Q” and “E” keys to these functions. What these functions do is tinker with the Target Arm Length setting in the Spring Arm component. Problem is when i’m in game and I pressed the binded keys, The engine locks up and crashes. What i’m trying to achieve is a zoom in and zoom out effect in a 2D top down game.

Any help would be greatly appreciated.
Kind regards.
Wes