Im having problems getting the cropped images url.

var mediaItem = @CurrentPage.baggrundsbillede;
var file = mediaItem.UmbracoFile;
var croppedUrl = Url.GetCropUrl(mediaItem, "bg");

@mediaItem
@file
@croppedUrl

@mediaItem successfully gives me the Id

@file successfully gives me the url of the original image

@croppedUrl fails and gives me the following error:

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS1973: 'System.Web.Mvc.UrlHelper' has no applicable method named 'GetCropUrl' but appears to have an extension method by that name. Extension methods cannot be dynamically dispatched. Consider casting the dynamic arguments or calling the extension method without the extension method syntax.

I've been copying from Umbracos own documentation: https://our.umbraco.org/documentation/getting-started/design/Rendering-Media/