image_rotate with ImageMagick: convert: unrecognized color

I keep getting error messages with image_rotate() using the ImageMagick toolkit:

(Messages partially in German, I guess you get it nevertheless)

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* ImageMagick-Kommando: /usr/bin/convert -rotate 90 -background 000000 -quality 100 -density 72 -units PixelsPerInch 'sites/default/files/img1.jpg' 'sites/default/files/img1.jpg'
* ImageMagick-Ausgabe:

ImageMagick hat einen Fehler zurückgegeben: convert: unrecognized color `000000'.

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I've been using the following code:

$node=node_load('103');
$source=$node->field_bild[0]['filepath']; 
$degrees='90';
return image_rotate($source, $source, $degrees);

where 'field_bild' is a imagefield. The same works well using the GD2 toolkit. Specifying a background color as a forth argument in image_rotate didn't succeed either.

(The image _will_ be rotated, though. Seems ImageMagick ignores the value.)

My ImageMagick version is ImageMagick 6.3.7 12/10/08 Q16.

Kind regards,

Felix