How do I get the primary namespace in an assembly?

For example, if I get the namespaces in EntityFramework.dll using

assembly.GetTypes().Select(t => t.Namespace).Where(n => n != null).Distinct() 

and then only list the namespaces with the minimum periods, I get the following two namespaces

System.Data.Entity
System.Linq.Expressions

However, System.Linq.Expressions is the namespace of an assembly that EntityFramework.dll is dependent on and not the primary namespace of EntityFramework.dll, which clearly is System.Data.Entity.

How do I distinguish between the 2?