Puzzling error?

[Mergon]

I am trying to create a simple adjustment that removes one skill proficiency and adds a different one.

The Component expressions are:
pCandExpr = component.BaseSkill & Helper.Proficient
pCandExpr2 = component.BaseSkill & !Helper.Proficient

The first is a selection of proficient skills on the hero.
The second is a selection of non-proficient skills on the hero.

My eval scripts is as follows:
Phase: First
Priority: 1000

~ If we're not enabled, get out now
doneif (field[pIsOn].value = 0)

doneif (field[pChosen].ischosen = 0)
doneif (field[pChosen2].ischosen = 0)

~ Mark our chosen skill as no longer proficient
perform field[pChosen].chosen.delete[Helper.Proficient]

~ Mark our chosen skill as proficient
perform field[pChosen2].chosen.assign[Helper.Proficient]

The basic script is stolen from the eval script on the Adjustment: Proficient Skill (pProfSki).
The key differences are for the removal of a skill proficiency at the same time as adding one.

When I chose to remove the Deception skill and add the Invistigate skill during a test, I get the following error . . .

Attempt to access pick information or behaviors for read-only thing 'skInvestig'

I've tried different Phases and Priorities:

Post-levels, 10000 & Final Phase, 10000 with the same issue.

It has to be a simple issue that I am missing because the original adjustment works fine.

[ShadowChemosh]

The 2nd custom expression on adjustments defaults to Things not Picks. And you can't modify Things only Picks.

Take a look at my "Weapon, Damage Type" adjustment I added. See where I modified the field pUsePicks2 to be 0 that is Things instead change that line of code to equal 1 for "Picks".