title: Assertion failed (blockSize % 2 == 1 && blockSize > 1) in cv::adaptiveThreshold
body: <p>I'm trying to do an adaptive threshold:</p>
  
  <pre><code>cv::Mat mat = cv::imread(inputFile);
  cv::cvtColor(mat, mat, CV_BGR2GRAY);
  cv::adaptiveThreshold(mat, mat, 255, cv::ADAPTIVE_THRESH_MEAN_C, cv::THRESH_BINARY, 20, 0);
  cv::imwrite(outputFile, mat);
  </code></pre>
  
  <p>But it fails with this message:</p>
  
  <pre><code>OpenCV Error: Assertion failed (blockSize % 2 == 1 &amp;&amp; blockSize &gt; 1) in cv::adaptiveThreshold, file ..\..\..\..\opencv\modules\imgproc\src\thresh.cpp, line 797
  </code></pre>
  
  <p>What is the problem?</p>