How come I always get wierd problems?

Ok, on a form I've created a complex custom "grid" in an app under VFP5. If a toggle is set on in a line on the grid, upon activation of the line the following code is executed (part of an EnableRow method):

TVar = "thisform.DescLine.ControlSource = S["+alltrim(str(CurrRow))+",18]"
&TVar
TVar = "thisform.InfoLine.ControlSource = S[" + alltrim(str(CurrRow))+",20]"
&TVar

The first one works just fine; the second blows up on the second &TVar line (consistently) with:
"The DataSource for this object must be a variable reference".

In this particular case (doesn't matter though) CurrRow = 1 and TVar is equal to the expected

"thisform.InfoLine.ControlSource = S[1,20]"

according to the debugger. Of course, S[1,20] is defined and equal to a short character string. To me this makes no sense because the first example works just fine. Any thoughts on what the heck VFP is doing?