c# - Look up table allowing null -

i have incident table has incidenttypeid(int) foreign key allowing null.

the incidenttype table has 2 columns id auto increment primary key not null , incidenttype not null nvarchar.

the incidenttype table populating ddl have listitem selected selected , value of "". checking if value "" , if setting selectedvalue of ddl null.

when select ddl fine , works ok.

when try , insert nothing selected , set null sqlexception on foreign key constraint on incidenttype id column not allowing null primary key .

how can round allowing null?

primary key cannot null. not possible entry must have id number. otherwise there noting create.