Using fonts with no isolated form

Hi,

I found a font which is lacking all the isolated forms (called 
TheMixArabic)

When I write a string with the character 0x0629 (Arabic Letter Teh 
Marbuta) fribidi returns the glyph 0xFE93 (Teh Marbuta Isolated Form).

Is there any way to force fribidi to use the alphabet character instead 
of the isolated form?

Thanks!