Entity Framework Error: Unable to track an entity of type OrderItem because primary key property ProductVariant is null

hi

in my OrderItems table i need a composite key as

Id, ProductCode,ProductVariant

I tested insert in sqlite with null ProductVariant but from webapi post

i recive error

Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NullableKeyIdentityMap<TKey>.Add(InternalEntityEntry entry)

How can solve?