The man page of "Xft" contains: 
 
man Xft>        XftFont * 
man Xft>        XftFontOpen (Display *dpy, int screen, ...); 
man Xft>        XftFontOpen takes a list of pattern elements of  the  form 
man Xft>        (field,  type,  value)  terminated  with a 0, matches that 
man Xft>        pattern against the available fonts and opens the matching 
man Xft>        font. 
 
i.e. the man page suggests to use 0. But on 64 bit platforms, 0 and 
NULL are not the same, therefore this causes problems and often 
crashes. I noticed this because "mlterm" used 0 instead of NULL in this 
function like written in the man page which made mlterm crash often 
on x86_64. To fix these crashes I only had to replace 0 -> NULL. 
 
The example in the man page 
 
man Xft>        Example: 
man Xft>             font   =   XftFontOpen   (dpy,  scr, 
man Xft>        XFT_FAMILY,  XftTypeString,   "charter", 
man Xft>        XFT_SIZE, XftTypeDouble, 12.0); 
 
doesn't have a terminating argument at all which is probably wrong as well.