diff --git a/src/machine.cpp b/src/machine.cpp
index ea96235..0274181 100644
--- a/src/machine.cpp
+++ b/src/machine.cpp
@@ -727,7 +727,7 @@ AsmResult Machine::assemble(Instruction const& instr)
     if (arg.mode == Mode::ZP_REL) {
         // Handle 65c02 bbrX & rmbX opcodes
         auto bit = arg.val >> 24;
-        arg.val = arg.val & 0xffffff;
+        arg.val &= 0xffffff;
         arg.opcode = arg.opcode + std::to_string(bit);
     }