Problem filling an image

I have some class that generates an image through GD.  I new the class 
for each image I create and I create thousand of images in the life of 
the program.  I have a problem where the background color is a random 
color if I set the background color to black (RGB 0,0,0).  It works if I 
set it to just about anything else (grey RGB 190,190,190 or linen 
250,240,230).  The actual fill color is black on the first image I 
generate (in the lifetime), then it will go random.  I have seen green, 
blue, red, purple, etc.  I assume this is a problem with GD, as it works 
with every other color I have tried except black.

Here is the code I am using.
     gdImagePtr _image;
    int _line_color;
    int style[20];

   _image = gdImageCreateTrueColor(xPixels, yPixels);

    // Allocate the first color
    gdImageColorAllocate(_image, backgroundColor.red(), 
backgroundColor.green(), backgroundColor.blue());

    // Fill the background
     _line_color = gdImageColorAllocate(_image, backgroundColor.red(), 
backgroundColor.green(), backgroundColor.blue());
    gdImageSetThickness(_image, 1);
    style[0] = _line_color;
    gdImageSetStyle(_image, style, 1);
    gdPoint* points = new gdPoint[4];
    points[0].x = xPixels;
    points[0].y = 0;
    points[1].x = xPixels;
    points[1].y = yPixels;
    points[2].x = 0;
    points[2].y = yPixels;
    points[3].x = 0;
    points[3].y = 0;
    gdImageFilledPolygon(_image, points, 4, _line_color);
    delete[] points;

Then I do all my other drawing.  I have tried it with and without the 
first color allocation (the 'Allocate the first color') which I needed 
when I was creating the image with gdImageCreate() which set the 
background color for me (and I didn't need to explicitly fill the 
background).

Any thoughts?

- Brian