MathML's msqrt not tall enough for fraction

Question

I am new to MathML so please bear with me. I created the following equation:

<math>
  <msqrt>
    <mfrac>
      <mi> A </mi>
      <mi> B </mi>
    </mfrac>
  </msqrt>
</math>

But the results show the sqrt only over "A". I want it to be taller so it is over A and B.

I tried playing around with changing the css file, like margins and padding, but nothing seemed to work.