I am having 2 classes and I created a many to many table. However I get an exception when trying to add data to this table. Here is my code. what am I doing wrong?

class Device:

private Set supportedFeatures = new HashSet();

/**
* @hibernate.set
* table="DEVICE_DEVICEFEATURE"
* cascade="all"
* inverse="false"
*
* @hibernate.collection-key
* column="DEVICE_ID"
*
* @hibernate.collection-many-to-many
* class="Feature"
* column="FEATURE_ID"
*
* @return the supportedFeatures.
*/
public Set getSupportedFeatures() {
return supportedFeatures;
}

//--------------------------------------------------
/**
* @param supportedFeatures The supportedFeatures to set.
*/
public void setSupportedFeatures(Set supportedFeatures) {
this.supportedFeatures = supportedFeatures;
}


Class Feature:

private Set associatedDevices = new HashSet();

/**
* @hibernate.set
* table="DEVICE_DEVICEFEATURE"
* cascade="all"
*
* @hibernate.collection-key
* column="FEATURE_ID"
*
* @hibernate.collection-many-to-many
* class="Device"
* column="DEVICE_ID"
*
* @return Returns the associatedDevices.
*/
public Set getAssociatedDevices() {
return associatedDevices;
}
/**
* @param associatedDevices The associatedDevices to set.
*/
public void setAssociatedDevices(Set associatedDevices) {
this.associatedDevices = associatedDevices;
}



The code for inserting new Device and new Feature to their table and to the many to many table:
Device objDevice;
objDevice = DeviceTest.createDevice();
insertObject(objDevice);


Feature objFeature;
objFeature = FeatureTest.createFeature();
insertObject(objFeature);

Set mySet = objFeature.getAssociatedDevices();
mySet.add(objDevice);
objFeature.setAssociatedDevices(mySet);


Set mySet1 = objDevice.getSupportedFeatures();
mySet1.add(objFeature);
objDevice.setSupportedFeatures(mySet1);

updateObject(objDevice);
updateObject(objFeature);

Running this code I get an Exception:
net.sf.hibernate.PropertyAccessException: IllegalArgumentException occurred while calling setter of Device.supportedFeatures
at net.sf.hibernate.property.BasicPropertyAccessor$BasicSetter.set(BasicPropertyAccessor.java:68)
at net.sf.hibernate.persister.AbstractEntityPersister.setPropertyValues(AbstractEntityPersister.java:222)
at net.sf.hibernate.impl.SessionImpl.doSave(SessionImpl.java:924)
at net.sf.hibernate.impl.SessionImpl.doSave(SessionImpl.java:857)
at net.sf.hibernate.impl.SessionImpl.saveWithGeneratedIdentifier(SessionImpl.java:779)
at net.sf.hibernate.impl.SessionImpl.save(SessionImpl.java:738)
at test.com.synchronica.cp.repository.RepositoryTest.insertObject(RepositoryTest.java:211)
at test.com.synchronica.cp.repository.FullTest.testInsert(FullTest.java:56)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at junit.framework.TestCase.runTest(TestCase.java:154)
at junit.framework.TestCase.runBare(TestCase.java:127)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:118)
at junit.framework.TestSuite.runTest(TestSuite.java:208)
at junit.framework.TestSuite.run(TestSuite.java:203)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:436)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:311)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: java.lang.IllegalArgumentException: argument type mismatch
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at net.sf.hibernate.property.BasicPropertyAccessor$BasicSetter.set(BasicPropertyAccessor.java:38)
... 22 more