diff --git a/cpu.js b/cpu.js
index 8974daa..7c0da07 100644
--- a/cpu.js
+++ b/cpu.js
@@ -338,6 +338,15 @@ async function stepCPU(debugInfo, debug = false) {
       CPU.currentInstruction.opcode = CPU.memory[CPU.IP];
       CPU.currentInstruction.operand = CPU.memory[CPU.IP+1];
       let executeInstruction = opcodes2mnemonics[CPU.currentInstruction.opcode];
+      if (typeof executeInstruction === 'undefined') {
+        let info = debugInfo[CPU.previousIP];
+        console.error();
+        console.error(`Error: Invalid opcode`);
+        console.error(`  Executing $${num2hex(info.machine[0])} $${num2hex(info.machine[1])}`);
+        console.error(`  from line ${info.lineNumber}: ${info.source}`);
+        console.error();
+        process.exit();
+      }
       executeInstruction(CPU.currentInstruction.operand);
       CPU.cycleCounter += 1;
     }