[metapost] "Improper transformation argument" Error

[Vafa Khalighi]

I have a metafont source file that has the following code:

begingroup
  my_T := currenttransform;
  currenttransform := identity slanted my_slant;
  currentpicture := nullpicture;
  clearxy;
  picture joined_fit_plus_pic;
  pickup pensquare;
  x1=x2=-2mag;  x3=x4=2mag+kashida_width;
  y1=y4=0;  y2=y3=horiz_width;
 fill (z1--z2--z3--z4--cycle) withweight 1;
  joined_fit_plus_pic := currentpicture;
  currenttransform := my_T;
endgroup;

when I run metafont on this, it runs smoothly without any problems.
but when I use mfplain format of metapost, I get the following error:

>> currenttransform.withweight1
! Improper transformation argument.
<to be read again>
                   ;
l.61  fill (z1--z2--z3--z4--cycle) withweight 1;

?


What is wrong? and how do I fix this? If required, I can provide the
full source.