Nested <tspan> elements not supported

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!-- Created with Inkscape (http://www.inkscape.org/) -->

<svg
   width="210mm"
   height="297mm"
   viewBox="0 0 210 297"
   version="1.1"
   id="svg1"
   xmlns:xlink="http://www.w3.org/1999/xlink"
   xmlns="http://www.w3.org/2000/svg"
   xmlns:svg="http://www.w3.org/2000/svg">
  <defs
     id="defs1">
    <linearGradient
       id="linearGradient1">
      <stop
         style="stop-color:#ff0000;stop-opacity:1;"
         offset="0"
         id="stop1" />
      <stop
         style="stop-color:#00ff00;stop-opacity:1;"
         offset="1"
         id="stop2" />
    </linearGradient>
    <rect
       x="150.02508"
       y="181.45326"
       width="412.12424"
       height="148.24613"
       id="rect1" />
    <linearGradient
       xlink:href="#linearGradient1"
       id="linearGradient2"
       x1="159.13997"
       y1="232.67492"
       x2="543.63868"
       y2="232.67492"
       gradientUnits="userSpaceOnUse" />
  </defs>
  <g
     id="layer1">
    <text
       xml:space="preserve"
       transform="scale(0.26458333)"
       id="text1"
       style="font-size:93.3333px;font-family:'Courier New';-inkscape-font-specification:'Courier New';white-space:pre;shape-inside:url(#rect1);display:inline;opacity:0.557377;fill:url(#linearGradient2);stroke-width:1.00157"><tspan
         x="150.02539"
         y="266.60392"
         id="tspan3"><tspan
           style="font-family:Cali;-inkscape-font-specification:Cali"
           id="tspan2">FOOBAR</tspan></tspan></text>
  </g>
</svg>