The following works fine (the ranges end up being null and the contains are false):
var wb = new XLWorkbook();
var ws = wb.AddWorksheet("Sheet1");
var range1 = ws.NamedRange("Bad");
var contains1 = ws.NamedRanges.Contains("Bad");
IXLNamedRange range2;
var contains2 = ws.NamedRanges.TryGetValue("Bad", out range2);