c# - Entity Framework NullReferenceException in Where -

i have following line of code calls entity framework , returning null reference exception.

list<fieldgroup> fieldgroups = datacontext.fieldgroups.where(fieldgroup => fieldgroup.field_catalog_id == fieldversion.fieldcatalogid && fieldgroup.field_catalog_version == fieldversion.fieldcatalogversion).tolist(); 

fieldversion being generated earlier in method , have confirmed it, , fields fieldcatalogid , fieldcatalogversion not null , have proper values.

my context, datacontext, while poorly named connecting database , have rows in fieldgroups table. have confirmed row field_catalog_id , field_catalog_version matching values being requested in exist should able find rows return.

i can't determine why line throwing nullreferenceexception. ideas appreciated. let me know if can provide more useful info.

edit: did take @ locals when exception being thrown , nothing being referenced showing null.

i added in debug.assert check datacontext, datacontext.fieldgroups, fieldversion, fieldversion.fieldcatalogid, , fieldversion.fieldcatalogversion not null before problem line. code got past 5 of asserts , still threw nullreferenceexception. can't find within fields null causing exception.