There's a check in ftobjs.c line 3345 : if ( face && face->charmap && face->num_glyphs )
1 - Check face pointer.
2 - Check charmap
3 - Check num_glyphs but in documentation charmap count is store on num_charmap...so it's strange...
It maybe a good idea to check this part of the code...
The code after use char index of 0 to get first glyph index, and the result was check using num_glyphs, the point 3 is maybe a copy\paste error...
Bye...