Imagick::annotateImage PHP causing Error Code 204 on GAE

Question

I redid a PHP GAE project of mine using the GD library to the Magick library. The updated code works fine in the GAE Launcher v1.9.23, however when deployed I'm getting in the browser:

Error: Server Error The server encountered an error and could not complete your request. Please try again in 30 seconds.

In the Google Dashboard under Monitoring→Logs, I'm getting:

A problem was encountered with the process that handled this request, causing it to exit. This is likely to cause a new process to be used for the next request to your application. (Error code 204)

I stripped out all the code and am still getting the error with just:

$draw = new ImagickDraw();
$draw->setFont( ABSPATH . 'banners/ttf_fonts/OpenSans-Semibold.ttf' );
$draw->setFillColor ( new ImagickPixel( 'white' ) );

$temp_img = new Imagick();
$temp_img->newImage( 100, 100, new ImagickPixel( 'green' ) );
$temp_img->annotateImage( $draw, 20, 20, 0, 'Hello world' );
$temp_img->setImageFormat('png');

header( 'Content-Type: image/png' );        
echo $temp_img;

Does GAE require a different approach for embedding text in an image?