Illegal forward reference

A symbol has been referred to that has not yet been defined, and a
directive or operator requires that its argument not be
forward-referenced. For example,

   IF MYSYM       ;error, MYSYM not defined yet 
       ;
       ENDIF
   MYSYM EQU 1 

Forward references may not be used in the argument to any of the IFxxx
directives, nor as the count in a DUP expression.