Generate Table using Javascript

I am trying to generate a table based on two loops I have running.

Right now I am just trying to get the table to generate, but keep getting this error:

Cannot read property 'appendChild' of null.

the line of code it is referring to is this:

body.appendChild(tbl);

but I'm not sure why tbl cannot be properly appended. Any help would be greatly appreciated.

var bmrMultiplier;
var userBMR;


function genderInput() {

  gender = prompt("Please enter your gender, note that answers are case sensitive and must be entered exactly as shown:\n\nF: for Female\nM: for Male\n");

  switch (gender) {
    case "M":
      console.log('Gender is ' + gender);
      bmrMultiplier = 26.4;
      weightInput();
      break;
    case "F":
      console.log('Gender is ' + gender);
      bmrMultiplier = 24.2;
      weightInput();
      break;
    default:
      alert('That is not a proper selection.');
      document.write('Please refresh the page to try again');
      console.log('default');
  }
}

//Sets up second user prompt and sets the value of difficulty based on the user selection

function weightInput() {

  weight = parseInt(prompt("Please enter your weight in kilograms"));

  console.log('Weight is ' + weight);

  if (isNaN(weight)) {
    console.log('This is not a number');
    alert('That is not a valid weight, please enter your weight in kilograms');
    weightInput();
  } else {
    console.log('This is a valid number. Your weight is ' + weight + ' kilograms.');
    calculateBMR();
    tableOutput();
  }

  function calculateBMR() {
    userBMR = weight * bmrMultiplier;
    console.log('Gender is ' + gender);
    console.log('Your weight is ' + weight + ' kilograms.');
    console.log("userBMR is " + userBMR);
  }

}


function tableOutput() {

  var body = document.body,
  tbl  = document.createElement('table');
  tbl.style.width  = '100px';
  tbl.style.border = '1px solid black';

  for (kph = 0; kph < 13; kph++) { 

    var tr = tbl.insertRow();

    for (hr = .25; hr < 2.75; hr = (hr + .25)) { 
      caloriesBurned = weight * kph * hr;
      console.log('kph is ' + kph + ' hours is ' + hr + ' calories burned is ' + caloriesBurned);

      var td = tr.insertCell();
      td.appendChild(document.createTextNode('Cell'));
      td.style.border = '1px solid black';


    }

  }

  body.appendChild(tbl);

}



genderInput();