Dissector only processes first packet

I'm writing my first dissector based on the example in the Developers Guide and README.developer.

I register my dissector for a certain port using

dissector_add_uint("udp.port", FOO_PORT, handle);

I notice that it only gets applied to the first packet that matches the port and I can't apply it to other packets, not even using "Decode As".

How can I figure out what the problem might be ?

--- Update ---

I noticed my first packet is marked as "malformed" by the sub-dissector I'm calling. When run on other packets it works. How can I make my dissector ignore such errors so it will be applied to the following packets ? Is there some error flag I need to clear ?