handler option for #toHTML not working

math.parse("sumMap(list,mapper)=sum(map(list,mapper))").toHTML({handler: (node, options) => "hello"})

'<span class="math-function">sumMap</span><span class="math-parenthesis math-round-parenthesis">(</span><span class="math-symbol math-parameter">list</span><span class="math-separator">,</span><span class="math-symbol math-parameter">mapper</span><span class="math-parenthesis math-round-parenthesis">)</span><span class="math-operator math-assignment-operator math-variable-assignment-operator math-binary-operator">=</span><span class="math-function">sum</span><span class="math-paranthesis math-round-parenthesis">(</span><span class="math-function">map</span><span class="math-paranthesis math-round-parenthesis">(</span><span class="math-symbol">list</span><span class="math-separator">,</span><span class="math-symbol">mapper</span><span class="math-paranthesis math-round-parenthesis">)</span><span class="math-paranthesis math-round-parenthesis">)</span>'

There is no "hello" in output. Am I doing something wrong here?