Can't get a skill function to pass a string in as a property name

In the APD command window with a skill prompt this works fine:

Skill > designDBID = axlDBGetDesign()

Skill > designDBID->prop->"EIGHTY_PERCENT_TEXT_BLOCK"

return value: "37"

But this doesn't work:

Skill > propName = "EIGHTY_PERCENT_TEXT_BLOCK"

Skill > designDBID->prop->propName

return value: nil

Basically I have a skill function that passes in a property name as a string, and I want to get the value of that property on a certain DB object, but it just won't work. What am I missing? As an example, this code won't work:

propName = "EIGHTY_PERCENT_TEXT_BLOCK"

designDBID = axlDBGetDesign()

print(designDBID->prop->propName)