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I think the changing of the property in the expression here is a red-herring - it won't work in either case. Update However, see at the end of my answer for some probably needlessly detailed exposition on why you didn't get an error first-time round. End Update You're using <code>ListBoxFor</code> - which is used to provide users with multiple selection capabilities - but you're trying to bind that to an <code>int</code> property - which cannot support multiple selection. (It needs to be an <code>IEnumerable<T></code> at least to be able to bind a list box to it by default in MVC) I think you mean to be using <code>DropDownListFor</code> - i.e. to display a list of items from which only one can be selected? If you're actually looking for single-selection semantics in a listbox, that's trickier to do in MVC because it's Html helpers are geared entirely around listboxes being for multiple selection. Someone else on SO has asked a question about how to get a dropdown to look like a list box: <a href="https://stackoverflow.com/questions/351483/how-do-i-create-a-listbox-in-asp-net-mvc-with-single-selection-mode">How do I create a ListBox in ASP.NET MVC with single selection mode?</a>. Or you could generate the HTML for such a listbox yourself. (Update) - Potentially needlessly detailed exposition(!) The reason you don't get an exception first time round is probably because there was no value for <code>id</code> in <code>ModelState</code> when the HTML was generated. Here's the reflected MVC source (from <code>SelectExtensions.SelectInternal</code>) that's of interest (the <code>GetSelectListWithDefaultValue</code> call at the end is the source of your exception): <pre><code>object obj = allowMultiple ? htmlHelper.GetModelStateValue(fullHtmlFieldName, typeof(string[])) : htmlHelper.GetModelStateValue(fullHtmlFieldName, typeof(string)); if (!flag && obj == null && !string.IsNullOrEmpty(name)) { obj = htmlHelper.ViewData.Eval(name); } if (obj != null) { selectList = SelectExtensions.GetSelectListWithDefaultValue(selectList, obj, allowMultiple); } </code></pre> Note first that the control variable <code>allowMultiple</code> is true in your case, because you've called <code>ListBoxFor</code>. <code>selectList</code> is the <code>SelectList</code> you create and pass as the second parameter. One of the things that MVC (unfortunately in some cases) does is to use <code>ModelState</code> to modify the select list you pass when re-displaying a view in order to ensure that values which were set in <code>ModelState</code> via a POST are re-selected when the view is reloaded (this is useful when page validation fails because you won't copy the values to your underlying model from <code>ModelState</code>, but the page should still show those values as being selected). So as you can see on the first line, the model's current value for the expression/field you pass is fished out of model state; either as a string array or as a string. If that fails (returns <code>null</code>)then it makes another go to execute the expression (or similar) to grab the model value. If it gets a non-null value from there, it calls <code>SelectExtensions.GetSelectListWithDefaultValue</code>. As I say - what you're trying to do will ultimately not work in either the case of <code>Id</code> or <code>SupplierId</code> (because they would need to be <code>IEnumerable</code>) but I believe this <code>ModelState</code>-><code>Eval</code> process is yielding a null value when you use <code>Id</code>, so the process of getting an 'adjusted' <code>SelectList</code> is skipped - so the exception doesn't get raised. The same is not true when you use <code>SupplierId</code> because I'll wager that there's either a value in <code>ModelState</code> at that point, or the <code>ViewData.Eval</code> successfully gets an integer value. Not throwing an exception is not the same as working!. End update
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