Using Sub Tables With Join

Hi.
I have a MSSQL DB with three tables: A, B, C.
C has a sub-table relation with B.
A & B have foreign-key relation as well.

If I create a query with B as the first table, and select C as a column, it works well.
But, if I create a query with A as the first table, inner join it with table B, and then try to use C as a column, I am getting a Object reference not set to an instance of an object. error.

Is such configuration not supported with sub tables?