revBrowserCallScript() not working in Mac OS X

[sp27]

My application (LC 4.6.3) does this:

revBrowserSet(sBrowserID, "htmltext", tMyHTML)

tMyHTML is a fully-formed Web page that includes this function:
function SetFontSize(FSizeStr) {
document.body.style.fontSize = FSizeStr; }

A button on the same card as the revBrowser instance does this:

revBrowserCallScript(sBrowserID, "SetFontSize", 16 & "px")

This works fine in Windows, but now that I started testing in Mac OS X (10.5.8. on PPC and 10.6.8 on Intel), the line above returns "execution error at line n/a (External handler: exception) near *** [WebUndefined cStringUsingEncoding:]: unrecognized selector sent to instance ...."

This is my second major revBrowser surprise since I started testing in Mac OS X. Am I on the wrong track? Does revBrowser work the same in Windows and Mac OS X? I realize the underlying browser comes from the OS. Do I need to do anything differently for revBrowser in Mac OS?

Thanks,

Slava