Value cannot be null. Parameter name: entity
<p>I have the following view (basic "edit" template)</p>

<p><div class="snippet" data-lang="js" data-hide="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>  @model SuccessStories.Models.Testimonials

@using (Html.BeginForm())
{
    @Html.AntiForgeryToken()
    
    &lt;div class="form-horizontal"&gt;
        &lt;h4&gt;Testimonials&lt;/h4&gt;
        &lt;hr /&gt;
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        @Html.HiddenFor(model =&gt; model.Id)

        &lt;div class="form-group"&gt;
            @Html.LabelFor(model =&gt; model.Testimonial, htmlAttributes: new { @class = "control-label col-md-2" })
            &lt;div class="col-md-10"&gt;
                @Html.EditorFor(model =&gt; model.Testimonial, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model =&gt; model.Testimonial, "", new { @class = "text-danger" })
            &lt;/div&gt;
        &lt;/div&gt;

        &lt;div class="form-group"&gt;
            &lt;div class="col-md-offset-2 col-md-10"&gt;
                &lt;input type="submit" value="Save" class="btn btn-default" /&gt;
            &lt;/div&gt;
        &lt;/div&gt;
    &lt;/div&gt;
}

&lt;div&gt;
    @Html.ActionLink("Back to List", "Index")
&lt;/div&gt;</code></pre>
</div>
</div>
</p>

<p>And the following actionresult in my controller:</p>

<pre><code>[HttpGet]
    public ActionResult Edit(int id)
    {
            TestimonialsContext testContext = new TestimonialsContext();
            Testimonials testimonials = testContext.testimonialContext.Find(id);
            return View(testimonials);  
    }

    [HttpPost]
    public ActionResult Edit(Testimonials testimonial)
    {
        TestimonialsContext testContext = new TestimonialsContext();
        testContext.Entry(testimonial).State = EntityState.Modified;
        testContext.SaveChanges();

        return RedirectToAction("Index");
    }
</code></pre>

<p>The error is on this line:</p>

<pre><code>testContext.Entry(testimonial).State = EntityState.Modified;
</code></pre>

<p>The error I get is "Value cannot be null.
Parameter name: entity</p>

<p>Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. </p>

<p>Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: entity"</p>

<p>Please help. I've looked this up but can't find a solution that would work for me. Thanks!</p>