Should I expect SAFE_HEAP to work with MEMORY64?

Hi, I'm trying to track down what looks like memory overwrite and am currently using 4.0.5. ASAN is not supported in this version but I can't see whether SAFE_HEAP is.
My reason for asking is that when I enable it, I get a call to segfault() in `runtime_safe_heap.js` very early on (far earlier than I would expect) with no other information.