Unexpected (for me) output when injecting new instructions

TLDR I built my own java bytecode modification framework for a project,
however I'm exploring ObjectWeb because it seems like an overall better
solution.

So I built a simple test, inject some junk, and didn't get what I expected
back.

I have a simple method to return true
public static boolean isTurkeyDay() {
    iconst_1
    ireturn
}

As a test I want to inject a nop after the ireturn

        public void visitInsn(int opcode) {
            switch (opcode) {
                case Opcodes.IRETURN:
                    mv.visitInsn(Opcodes.IRETURN);
                    mv.visitInsn(Opcodes.NOP);
                    break;
                default:
                    super.visitInsn(opcode);
            }

I expect:
public static boolean isTurkeyDay() {
    iconst_1
    ireturn
    nop
}

However I get

public static boolean isTurkeyDay() {
    iconst_1
    ireturn
    athrow
}

What am I missing here?

Thank you for this


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