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Congrid problem

I'm running IDL 5.3 on HP-UX and seeing the following
problem with congrid.  I have a byte image that has large areas
of a particular solid color (a background color).
I rescale the image to be larger using congrid with the
/interp flag set.  My nice solid areas of the background color,
which I would still expect to see unspoiled, now have artifacts
in them with value background_color - 1.  This looks like a
bug.  What's going on?

Here's an example that show's the problem:

IDL> a = bytarr(100,100) + 155b
IDL> b = congrid(a,140,140,/interp)
IDL> print, min(b), max(b)
 154 155
IDL> w = where(b eq 154, count)
IDL> print,count
IDL> tvscl, b eq 154   ;shows strange pattern of lines and dots

I would expect b in this example to be filled only with value 155.
Is this a bug or is there some other reason I am seeing this behavior?

BTW: I see the exact same behavior with INT instead of BYTE.