diff --git a/lib/opcode.js b/lib/opcode.js
index 417213d..04c3ccf 100644
--- a/lib/opcode.js
+++ b/lib/opcode.js
@@ -33,7 +33,10 @@ Opcode.prototype.fromString = function(str) {
 };
 
 Opcode.prototype.toString = function() {
-  return Opcode.reverseMap[this.num];
+  var str = Opcode.reverseMap[this.num];
+  if (typeof str === 'undefined')
+    throw new Error('Opcode does not have a string representation');
+  return str;
 };
 
 Opcode.map = {