wp.getComments is returning nill, when i called from my iphone app

i have called wp.getComments from my iphone app, but its returning nil,,

following r my parameters can anybody tell me where i gone wrong

NSDictionary *DicCommentStructure = [NSDictionary dictionaryWithObjectsAndKeys:@"",@"Approved",@"50",@"10",nil];

    // Send XMLRPCRequest to the server
    NSArray *arrParameters = [NSArray arrayWithObjects:@"1", @"username", @"password",DicCommentStructure, nil];
    NSString *strServer = [[[NSString alloc] initWithString:@"http://xyz.com/xmlrpc.php"] autorelease];         // the server
    NSString *strMethod = [[[NSString alloc] initWithString:@"wp.getComments"] autorelease];                        // the method
    XMLRPCRequest *objXMLRPCRequest = [[XMLRPCRequest alloc] initWithHost:[NSURL URLWithString:strServer]];
    [objXMLRPCRequest setMethod:strMethod withObjects:arrParameters];

thanx in advance!!