title: Flux fieldKeys() is type nil, c# querying
body: <p>I'm working in C# and I'm trying to get the schema of my database, but can't call the function FieldKeys because of the error:
  <code>error calling function &quot;FieldKeys&quot;: cannot call function, value is of type nil</code></p>
  <p>Im calling it like this, other methods like <code>measurements()</code> were fine, only this one is messy</p>
  <pre><code>...
  StringBuilder query = new($&quot;import \&quot;influxdata/influxdb/v1\&quot;\n&quot; +
                            $&quot;v1.fieldKeys(bucket: \&quot;{database}/{_retentionPolicy}\&quot;)&quot;);
      
  var fluxTables = await client.GetQueryApi().QueryAsync(query.ToString());
  </code></pre>
  <p>I've tried QueryRaw which throws the same error.</p>