"Invalid use of a reserved word" in a script

[Duggan]

This is the code I'm using:

<eval index="1" phase="PreTraits" priority="4900">
<before name="Calc trtFinal"/><![CDATA[
~build the candidate expression out of our Skill and Skills tags
var skills as string
skills = tagids[Skill.?, " | "]
var skilltypes as string
skilltypes = tagids[Skills.?, " | "]

var combined as string
if (tagis[Skills.?] <> 0) then
if (tagis[Skill.?] <> 0) then
combined = skills & " | " & skilltypes
else
combined = skills
endif
else
combined = skilltypes
endif

if (empty(combined) <> 0)
field[usrCandid1].text &= " & (" & combined & ")"
endif

perform field[usrChosen1].chosen.field[trtBG].modify[+,2,"Background"]
]]></eval>

It's giving me an error of

Syntax error in 'eval' script for Thing 'fRPlusSkMx' (Eval Script '#1') on line 19
-> Invalid use of a reserved word in script

I've tried replacing "empty" with a check for the length of the string, but to little avail. Are these not the correct keywords for that function? If I comment out the "if" statement and the "endif", it works. I suppose that, if all else fails, I can rig up if statements with invoking tagis again to only do that check if either tag is present, but I'd prefer to avoid that.