title: truffle debugger: TypeError: Cannot read property 'line' of undefined
body: <p>I am trying to use Truffle 4.1.0 to debug Solidity code.
  Debugger starts up properly, displaying the first lines, but you can't step through contract!</p>
  
  <p>The full stacktrace of the error as shown at the Terminal is:</p>
  
  <pre><code> TypeError: Cannot read property 'line' of undefined
       at Object.formatRangeLines (/usr/local/lib/node_modules/truffle/build/webpack:/~/truffle-debug-utils/index.js:201:1)
       at printState (/usr/local/lib/node_modules/truffle/build/webpack:/~/truffle-core/lib/commands/debug.js:125:1)
       at Object.interpreter (/usr/local/lib/node_modules/truffle/build/webpack:/~/truffle-core/lib/commands/debug.js:404:1)
       at ReplManager.interpret (/usr/local/lib/node_modules/truffle/build/webpack:/~/truffle-core/lib/repl.js:119:1)
       at bound (domain.js:280:14)
       at REPLServer.runBound [as eval] (domain.js:293:12)
       at REPLServer.&lt;anonymous&gt; (repl.js:539:10)
       at emitOne (events.js:96:13)
       at REPLServer.emit (events.js:188:7)
       at REPLServer.Interface._onLine (readline.js:232:10)
       at REPLServer.Interface._line (readline.js:583:8)
       at REPLServer.Interface._ttyWrite (readline.js:860:14)
       at REPLServer.self._ttyWrite (repl.js:612:7)
       at ReadStream.onkeypress (readline.js:119:10)
       at emitTwo (events.js:106:13)
       at ReadStream.emit (events.js:191:7)
       at emitKeys (internal/readline.js:389:14)
       at next (native)
       at ReadStream.onData (readline.js:970:36)
       at emitOne (events.js:96:13)
       at ReadStream.emit (events.js:188:7)
       at readableAddChunk (_stream_readable.js:176:18)
       at ReadStream.Readable.push (_stream_readable.js:134:10)
       at TTY.onread (net.js:547:20)
  </code></pre>
  
  <p>Digging into method <code>formatRangeLines: function(source, range, contextBefore)</code>
  When <code>console.log(range);</code> it prints:</p>
  
  <pre><code>{ start: undefined, end: undefined }
  </code></pre>
  
  <p>Actually, the exception raised when accessing <code>range.start</code> at this block:</p>
  
  <pre><code>var startBeforeIndex = Math.max(
    range.start.line - contextBefore, 0
  );
  </code></pre>
  
  <p>Any suggestions?
  Thanks,</p>