How to call a function correctly in script.js for a Chrome extension?

A simple extension, I just want to select a text and search it on a specific website by pressing Alt-Z.

Here is my manifest.js:

{
    "name": "Jisho megasearch",
    "version": "1.0",
    "manifest_version": 2,
    "description": "my megadescription",

    "background": { 
      "scripts": ["script.js"]
    },
    "commands": {

          "commence_megasearch": {
            "suggested_key": {
                "default": "Alt+Z"
            },
            "description": "commences megasearch"
          }       
        }
}

Here is the script.js:

function getword(info,tab) {
    chrome.tabs.create({  
    url: "http://jisho.org/search/" + info.selectionText,
  });           
}
chrome.commands.onCommand.addListener(function(command) {
    if (command == 'commence_megasearch')
        getword();

});

I get the error: Error in event handler for commands.onCommand: TypeError: Cannot read property ‘selectionText’ of undefined at getword.

Do I call getword() incorrectly?
I’m following this example here (don’t need the actual context menu):

function getword(info,tab) {
  //console.log("Word " + info.selectionText + " was clicked.");
  chrome.tabs.create({  
    url: "http://jisho.org/search/" + info.selectionText,
  });           
}

chrome.contextMenus.create({
  title: "Search: %s", 
  contexts:["selection"], 
  onclick: getword,
});