Within an UmbracoApiController, I'm trying to use GetCropUrl on a dynamic object with an ImageCropper property.

Using GetCropUrl item.GetCropUrl("Image", "FullWidth"); returns the crop as expected.

Using an exteneded version of GetCropUrl item.GetCropUrl(propertyAlias: "Image", cropAlias: "FullWidth", useCropDimensions: true, ratioMode: ImageCropRatioMode.Height, furtherOptions: "&slimmage=true" ); returns an empty string.

public class CategoryApiController : UmbracoApiController {
  ...
  int rootId = Umbraco.TypedContentAtRoot().First().Id;
  var root = Umbraco.Content(rootId);
  var categories = rootNode.Descendants("CategoryContainer").First();
  var items = categories.Descendants("CategoryItem").Where("Visible");
  ...
  foreach (var item in Items) {
    if (item .HasValue("Image")){
      img = TipItem.GetCropUrl(
        propertyAlias: "Image", 
        cropAlias: "FullWidth", 
        useCropDimensions: true, 
        ratioMode: ImageCropRatioMode.Height, 
        furtherOptions: "&slimmage=true" );
    }
  }
}

Any ideas why would be greatly appreciated.

Umbraco Version 7.2.1