Container Query units - vertical unit all evaluate to zero

I think I've fallen into a pitfall with container query units. 100cqb and 100cqh are evaluating to zero, but 100cqi and 100cqw are working as expected.

Here's a simplified version I want I believe is all my relevant css:

.parent {
  container-name: foo;
  container-type: size;
  min-height: 500px;
  display: block;
}
.child {
  height: 200px;
  min-height: 100px;
  max-height: calc(100cqh - 64px);
  resize: vertical;
}


Any ideas to the cause of why my max-height is coming out as 0px instead of >= 436px?
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