Field's getRelationshipOrder() is always 0 or 1

I got an apex class that is used for determining whether a particular field of an object is part of a master-detail or a lookup relationship. It utilizes getRelationshipOrder() which has possible values {1, 0, null}.
Sample apex code where CR1__c, CR2__c are custom objects where CR2__c.CR1_CR2__c is involved in a master-detail relationship with CR1 and CR1__c.CR2_CR1_Lookup__c is involved in a lookup with CR2__c.

@isTest
public class Scratchpad {

static testMethod void printMessage(){
System.debug('Hello world!');
System.debug(Opportunity.AccountId.getDescribe().getRelationshipOrder());
System.debug(CR1__c.CR2_CR1_Lookup__c.getDescribe().getRelationshipOrder());
System.debug(CR2__c.CR1_CR2__c.getDescribe().getRelationshipOrder());
}

}
The above snippet yields the output as:
'Hello world!'
null
null
0

When the same equivalent code is run in Force's wsc connector SOAP API, it is displayed as:
'Hello world!'
0
0
0
Checking the source, Field.class
```
private int relationshipOrder;

@Override
public int getRelationshipOrder() {
return relationshipOrder;
}

@Override
public void setRelationshipOrder(int relationshipOrder) {
this.relationshipOrder = relationshipOrder;
relationshipOrder__is_set = true;
}
```
is the root cause that can never have null. Can this be fixed to accommodate what the apex does.