Bing API Image Search Filter

I am using Bing Sharp 2.0. I would like to search for images by filters, so I follow the codes in the sample:

SearchRequest searchRequest = new SearchRequest { AppId = appId, Query = query, Market = "en-US" };
ImageRequest imageRequest = new ImageRequest();
imageRequest.Filters = buildFilterArray();
imageRequest.Count = imageCount;
imageRequest.Offset = (imageCount * pageNumber);
ImageResponse response = API.Image(searchRequest, imageRequest);

And this is buildFilterArray:

private string[] BuildFilterArray()
{
    List<string> filters = new List<string>();
    filters.Add("Size:Small");
    filters.Add("Size:Medium");
    return filters.ToArray();
}

But the result does not return anything (response.Total == 0).

I found the reason because I defined 2 filters in buildFilterArray (), while I delete one (no matter which), my search returns with the expected results.

This is also confirmed by issuing an HTTP request directly from IE, this returns the results:

http://api.search.live.net/xml.aspx?AppId=0B409D9BA6759BCD3CC3D8B9A6A90F2907274BC7&Query=tektronix&Version=2.0&Market=en-US&Sources=Image&Image.Count=10&Image.Offset=0&Image.Filters=Size:Small

So far this is not:

http://api.search.live.net/xml.aspx?AppId=0B409D9BA6759BCD3CC3D8B9A6A90F2907274BC7&Query=tektronix&Version=2.0&Market=en-US&Sources=Image&Image.Count=10&Image.Offset=0&Image.Filters=Size:Medium&Image.Filters=Size:Small

Am I doing something wrong? How can I put multiple filters in a search query?