Increment variable in Script Task

I have a variable called varChangeVersion of type Int32 and package level scope and a default value of 0. 

I am using a Script Task to increment this variable by 1 each time the SSIS package runs (line 1). I am using the get current version line for debugging purposes.

I have added the varChangeVersion variable to the ReadWriteVariables list.

If I run run the package I can see the varChangeVersion always changing to 1.

It does not seem to be writing the new version. What I expect to see is an increment of 1 each time the package runs so 0, 1, 2, 3, 4 etc.

Am I missing something?