EasyMock incompatible return value type for return type Collection

EasyMock incompatible return value type for return type Collection

i am getting the following exception for a method where the return type of
the method is collection...
java.lang.IllegalStateException: incompatible return value type at
org.easymock.internal.MocksControl.andReturn(MocksControl.java:218)
expect(training.getTrainingMaterials()).andReturn(new HashSet());
where the training.getTrainingMaterials() --> return type is Collection..
Can you please help me what needs to be done?
Thanks.