gdImageFilledPolygon no-op

I'm reading in a file containing vertex coordinates as well as a single
data value for various six-sided polygons.  The data value is converted
into a color using gdImageColorResolve and is returning a valid index.
However, gdImageFilledPolygon appears to be a no-op.  I can comment out
the polygon function with no noticeable change in execution time for the
entire input file.  The image I get is entirely black, which is the
default fill for a True Color image.

I have verified the input values to gdImageFilledPolygon.  The gdPoint
array is always a length of 6 due to my input files, but the gdPoint
array is dynamic so it would handle other polygon shapes as well.

I honestly can't see what I'm doing different from example code on the
website and within the source, other than that I'm creating the gdPoint
array using malloc rather than on the stack.

Any thoughts?

Jeff