Hi,
Two years too late, but I just wanted to corroborate and expand on DavidMurray_ODWC 's solution. My problem was also caused by customization, specifically custom scripts in my default toolbox.
I had previously changed the default Toolbox.tbx to a custom toolbox. I fixed the error by adding the standard Toolbox.tbx back to my project and making it the default, then removing my custom toolbox (just to be safe). I then tried packaging same project with my custom toolbox as a secondary, non-default toolbox and it worked fine. I am using 2.9.0.
As a note, this error occurred whether or not I had the "Include Toolboxes" parameter checked.
Best,
Macy