Invalid expression

[BackWorld]

DDXMLElement *root = [document rootElement];
    DDXMLNode *defaultNameSpace = [root namespaceForPrefix:@""];
    defaultNameSpace.name = @"default";
    int index = 1;
    NSError *error;
    
    NSArray *spineNodes = [root nodesForXPath:@"//default:ncx/default:navMap" error:&error];

why?

[BackWorld]

DDXMLErrorDomain Code=1207