The error that shows in Open to LAN "Not a valid port. Leave the edit box empty or enter a number between 1024 and 65535." doesn't show if you enter a number less than 1024.

It shows if the input is either not a number, greater than 65535, or zero. 

Based on the wording, I would assume that any number that is less than 1024 would also not work, but they do (excluding zero): "number between 1024 and 65535"