How to debug INT calls in BOCHS?

In NOS, I have set up an API on INT 21 for use by the programmer/user to perform basic tasks without long and complicated code. However, as of the moment, I am having issues getting this API called from the kernel. I have done several step-throughs with Bochs, to no avail. I cannot seem to figure out what is wrong with my code. As far as I can figure out, the API's resident address in RAM (0x8000:0x0000) is getting into the IVT correctly, and the API is getting put in the correct spot, but when I perform an INT 0x21, Bochs seems to "get lost", i.e. it doesn't execute the API code, but whatever it does execute has a LOCK prefix that isn't supposed to be there and it locks up the "processor". It does this every time, I have tried swapping the values in the IVT, I've done memory dumps and the API is always at 0x8000:0x0000 like it's supposed to be. I have an issue on GitHub if you want more info: https://github.com/nkeck720/nos/issues/38

I haven't tested this on real hardware, but VirtualBox and DosBox both seem to do the same thing.

It always fails at the "test call", where I check to make sure that the API is installed correctly:

Code:

        ; We need to be sure that INT 21 works. Function 00 is an install
        ; check, and so we will call it with that value.
        mov ah, 00h
        int 21h
        ; If AX=5555h, we are done here.
        cmp ax, 5555h
        jne api_load_error

As far as I can tell, it never makes it to the CMP instruction, and I can't tell where on earth Bochs went off to because its builtin debugger doesn't trace interrupt calls. all I get is this:

I'm at a loss here. Can someone help me out with this?

EDIT: Yes, I have made sure ES=0x0000, so in effect my code is pointing at 0x0000:0x0086 and 0x0000:0x0084 in the IVT.