The error happens because there is no version available for the crawler. you need to add the following nodes to capabilities:

      <capability name="majorversion" value="0" />
      <capability name="minorversion" value=".0" />
      <capability name="version" value="0.0" />

(or other applicable versions...)

A better solution would be to add a version for your crawler in its user agent. Ideally, if your crawlers user agent adheres to the regex in the general crawler definition, you would not need to create your own capabilities file. The default definition is this:

  <browser id="DefaultCrawler" parentID="Default">
    <identification>
      <userAgent nonMatch="(?i-s:crawler|bot|spider|larbin|search|indexer|archiver|Nutch)" />
      <capability name="isMobileDevice" nonMatch="(?i-s:true)" />      
      <userAgent match="(?'browser'[^/]*)/(?'version'(?'major'\d+)(?'minor'\.\d+)\w*)|(?'browser'^.*)[ /](?'version'(?'major'\d+)(?'minor'\.\d+)\w*)" />     
    </identification>
    <capabilities>
      <capability name="crawler" value="true" />
      <capability name="browser" value="${browser}" />
      <capability name="majorversion" value="${major}" />
      <capability name="minorversion" value="${minor}" />
      <capability name="version" value="${version}" />
      <capability name="tagWriter" value="System.Web.UI.HtmlTextWriter" />
    </capabilities>
  </browser>

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