DataNucleus JDO primary key with enum - datanucleus

Question

I am attempting to create a primary key for my class using the example at http://www.datanucleus.org/products/accessplatform_4_0/jdo/orm/compound_identity.html as a guide. My primary key consists of 3 fields, 2 Strings and 1 enum.
According to http://www.datanucleus.org/products/accessplatform_4_0/jdo/types.html, an enum should be able to be part of the primary key, but when the enhancer runs, I get the following:
Caused by: org.datanucleus.metadata.InvalidPrimaryKeyException: Class "A" has been specified with an object-id class A$PK which has 2 fields, whereas the class has been defined with 3 fields in the primary key. The number of fields in the objectidClass and the number in the primary key must concur.
Can someone clarify as to whether the BEnum enum (just a standard enum with no extra methods) can be used as a field in the primary key? If not, what would be the suggested workaround? If it can, what am I doing wrong?
The class is defined as:
PersistenceCapable(detachable = "true", identityType = IdentityType.APPLICATION, objectIdClass = A.PK.class)
public final class A implements Serializable {
#PrimaryKey
private String field1;
#PrimaryKey
private BEnum field2;
#PrimaryKey
private String field3;
public static class PK implements Serializable {
private static final long serialVersionUID = 1L;
public String field1;
public BEnum field2;
public String field3;
public PK() {
}
public PK( String s ) {
StringTokenizer token = new StringTokenizer( s, ":" );
this.field1 = token.nextToken();
this.field2 = BEnum.valueOf( token.nextToken() );
this.field3 = token.nextToken();
}
#Override
public String toString() {
return field1 + ":" + field2 + ":" + field3;
}
#Override
public int hashCode() {
int hash = 7;
hash = 67 * hash + Objects.hashCode( this.field1 );
hash = 67 * hash + Objects.hashCode( this.field2 );
hash = 67 * hash + Objects.hashCode( this.field3 );
return hash;
}
#Override
public boolean equals( Object obj ) {
if( obj != null && (obj instanceof PK) ) {
PK that = (PK)obj;
return Objects.equals( this.field1, that.field2 ) && this.field2 == that.field2 && Objects.equals( this.field3, that.field3 );
}
return false;
}
#Persistent
public String getField1() {
return field1;
}
public void setField1(String field1) {
this.field1 = field1;
}
}
The enum is simply:
public enum BEnum {
VALUE1, VALUE2, VALUE3, VALUE4
}