EObjectAtOffsetHelper throws NPE with Fragments and AST rewrite

we have the following grammar

Model:
	elements+=Element*
;

Call:
	"call" name=ID
;

Element:
	Call | OtherCall
;

// if i replace this with v1=ID "=" v2=ID it works
CallParameter returns Expression:
    PathExpression (
          {Assignment.assignee=current} '='
          value=ID
    )
;


PathExpression:
	path=ID
;

OtherCall:
     {OtherCall}
    'other' SomeDefinition;

fragment SomeDefinition returns Definition:
    simpleName=ID
   '{'
            (parameters+=CallParameter ';')*
            (   'target' '='
                    ( targets+=[Call]
                    ) ';'
                (parameters+=CallParameter ';')*
            )
        '}'
    
;

the following unit test show the bad behavior of EObjectAtOffsetHelper

@ExtendWith(InjectionExtension)
@InjectWith(MyDslInjectorProvider)
class MyDslParsingTest {
	
	@Inject
	ParseHelper<Model> parseHelper
	
	@Inject
	EObjectAtOffsetHelper objectAtOffsetHelper
	
	@Test
	def void loadModel() {
		val model = '''
			call Call1
			call Call2
			other Other1 {
				x=y;
				target=Call1;
			}
		'''
		val result = parseHelper.parse(model)
		Assertions.assertNotNull(result)
		val errors = result.eResource.errors
		Assertions.assertTrue(errors.isEmpty, '''Unexpected errors: ?errors.join(", ")?''')
		val index = model.lastIndexOf("Call1")
		val o = objectAtOffsetHelper.resolveCrossReferencedElementAt(result.eResource as XtextResource, index+1)
		println(o)
		// should return Call1
		// throws NPE
	}
}

java.lang.NullPointerException
	at org.eclipse.xtext.resource.EObjectAtOffsetHelper.resolveCrossReferencedElement(EObjectAtOffsetHelper.java:139)
	at org.eclipse.xtext.resource.EObjectAtOffsetHelper.getCrossReferencedElement(EObjectAtOffsetHelper.java:84)
	at org.eclipse.xtext.resource.EObjectAtOffsetHelper.resolveCrossReferencedElementAt(EObjectAtOffsetHelper.java:53)
	at org.xtext.example.mydsl2.tests.MyDslParsingTest.loadModel(MyDslParsingTest.java:62)

workaround:
inline the fragment