No There is nothing wrong with Nullable FKs. This is common when the entity the FK points to is in a (zero or one) to (1 or many) relationship with the primary Key referenced table.
An example might be if you had both a Physical address and a Mailing address attribute (column) in a table, with FKs to an Address table. You might make the Physical address nullable to handle when the entity only has a post office box (mailing address), and the mailing address nullable to handle when the mailing address is the same as the physical address (or not).