title: What is this Azure-Search error about "invalid expression" on $filter
body: <p>When using both C# .NET code to send a query to Azure Search and when using the "Search Explorer" within the Resource-Manager style Azure Portal, Azure search will succeed when given the query (as input in Search Explorer):</p>
  
  <p><code>search=foo&amp;facet=category&amp;$filter=category eq 'Teams'</code></p>
  
  <p>But fail when given:</p>
  
  <p><code>search=foo&amp;facet=categoryDetails&amp;$filter=categoryDetails eq 'Subcategory'</code></p>
  
  <p>The error returned is:</p>
  
  <pre><code>{
      "error": {
          "code": "",
          "message": "Invalid expression: The operand for a binary operator
                     'Equal' is not a single value. Binary operators require
                     both operands to be single values.\r\nParameter name:
                     $filter"
      }
  }
  </code></pre>
  
  <p>(Message is a single-line. It's formatted with line breaks here for easier reading.)</p>
  
  <p>I even tried with another facet whose name is also a camelcase term, and the search also hit the same error.</p>
  
  <p>My theory is that camelcase names cause issues with Azure Search. Has anyone else experienced this before? I'll suppose I'll experiment with all lowercase names while waiting for light to be shed on this.</p>