Catching Errors from WebAssembly in an Optimized Code

I think I need some human input here :) I have a simple CPP code:

#include <stdexcept>

#include <emscripten/bind.h>

using namespace emscripten;

double add(double a, double b) 
{
    if(a>5)
        throw std::runtime_error("big a");
    return a+b;
}

EMSCRIPTEN_BINDINGS(my_module) {
    function("add", &add);
}

I compile this simple example with:

emcc -lembind -o quick_example.js quick_example.cpp -fexceptions -sDISABLE_EXCEPTION_CATCHING=0 -O2

When I catch the error in JavaScript, it prints out weird numbers instead of the error message. I can only see the error message when not using any level of optimization. What am I missing here.