Operating system: Linux-4.15.0-51-generic-x86_64-with-debian-stretch-sid 64 Bits
Graphics card: GeForce GTX 970/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 384.130
Broken: version: 2.80 (sub 74), branch: blender2.7, commit date: 2019-06-13 19:32, hash: rB5e626e766459
Short description of error
after changing the "value" field manually, the value becomes equal to 0, and is no longer controlled by the driver.
Exact steps for others to reproduce the error
the "aux.arm_elbow.L" shape_key-driver is broken in the attached file.