SSIS Variable not changing through script.

I have created a Script Task in a data flow, in which for debugging purposes, I have done the following:

    public override void PostExecute()
    {
        base.PostExecute();
        /*
         * Add your code here
         */
        this.Variables.maxNum = 55;
    }

maxNum in this case is an int32 variable with package-wide scope and starting value 0.

After the script task, I have a Derived Column task which creates a new column for each row containing the variable, but the variable is always 0. Is there anything I am doing wrong?