Can the structure, having few integers and 'opaque', be manipulated upon?

{ i32, i32, opaque }

When I try running through JIT the code, incrementing the field in such structure, I am getting an assert:
Assertion failed: (Ty->isSized() && "`!"), function getAlignment, file /tmp/llvm-build/2.7/llvm/lib/Target/TargetData.cpp, line 498.

I understand that C/C++ can't possibly produce such structure due to their language limitations. But llvm is supposed to be more generic. There should be no problem manipulating such structure, since opaque is only in the end.