Lookup field problem

hello i have a lookup field that use a table to get values, this work fine but i select allow add for user add the value if not exist, the problem is when input the name on add and click add system show:Insert failedadding direct in table work fine

any idea/

uhhh now i notice that any ADD in lookup table work, are simple table just the ID and documentName so just 2 fields, Documentname is the lookup the values