[metapost] Cannot draw line at specified direction

[Yasir Malik]

Hello,

I am unable to draw a line at the angle specified in the dir() function. 
Here's what I'm doing:
beginfig(1);
u = 1cm;

pair C, D;

cdLen = 4u;

C = (3u, 0);
D = cdLen*dir(91);

draw C--D;

endfig;


The line is not at 91 degrees.  Even 110 does not produce the correct 
angle.  However, if I choose 179, the angle seems to be correct.  If 179 
works, I would expect 91 and 110 to work because they are all in the same 
quadrant.

What am I missing?

Thanks,
Yasir