Error 1 in Mapping Entity Framework

Hi,

i wrote an Entity Framework Application with one table "Test".
In this table is an attribute "Type". If the the is 1 I will map to 
DerivedClass1
if the attribute is 2 I will map to DerivedClass2.
I designed my model and everything works fine.
But I still habe an error in VS ErrorTab:

Error	1	Entity types DerivedClassType1, DerivedClassType2 are mapped to 
table Test without discriminating conditions on all mappings to the table.

What does this mean ?

Thanx 
Stephan