inline image filter doesn't work?

I'm using img_assist-5.x-1.4 with drupal-5.2

As soon as I set the image height I get an empty span environment and so no image displayed at all.

Examples:

[img_assist|nid=10|title=|desc=|link=none|align=left|width=640|height=204]

filters to

<span class="inline left"></span>

[img_assist|nid=10|title=|desc=|link=none|align=left|width=640|height=]

filters to

<span class="inline left"><img src="mypath/images/image.thumbnail.jpg" alt="" title="" class="image thumbnail" width="100" height="32" /></span>

showing the thumbnail image.

[img_assist|nid=10|title=|desc=|link=none|align=left|width=100|height=32]

filters to the empty span environment as mentioned above.

All image sizes get scaled properly and are found residing in the image directory.