Invalid visual variable for the max-width property of a style. In MVC asp.net

I have the following code:

<% For Each img In Model.listadoNoticias.Item(0).FOTO
       Dim id_LaImagen = "foto_" & img.IdFoto.ToString()
       Dim src_imagen = img.RutaFoto.Split("/")
       Dim ruta_Foto = Server.MapPath("~") & src_imagen(2) & "\" & src_imagen(3) & "\" _
       & src_imagen(4) & "\" & src_imagen(5) & "\" & src_imagen(6) & "\" & src_imagen(7)
       Dim fs As System.IO.FileStream = New System.IO.FileStream(ruta_Foto.ToString(), System.IO.FileMode.Open, System.IO.FileAccess.Read, System.IO.FileShare.Read)
       Dim LaImagen As System.Drawing.Image
       LaImagen = System.Drawing.Image.FromStream(fs)
       Dim width_original = LaImagen.Width
       Dim height_original = LaImagen.Height
       Dim proporcion = width_original / height_original
       Dim max_width = 380 * proporcion & "px"
  %>
<li>                                                                                           
    <img alt="" id="foto_<%:img.IdFoto.ToString()%>"  
    src="<%:Html.Action("FotoNoticiaString", "Noticias", New With {.id = img.IdFoto}).ToString()%>" 
    style="max-height:380px;max-width:<%:max_width%>" class="img-responsive centrado_carrusel"/>                                           
</li>

Which I show images in a Carousel Slider, in which I obtain the original dimensions to calculate its proportion, defining a fixed height, and with the proportion obtain the width or maximum width.

I store the value of the width ( Dim max_width) to assign it to the style of the image, in this case to max-width, but it tells me that the value is not valid.

I would like to know what would be the correct way to assign the value of the Visual to a property of the style of the image. When it is a property that is not of the style, for example assigning it an id, class, etc. visually, it does not indicate the warning, but in this case it does.

Here I show a picture of what happens