diff --git a/lib/opcode.js b/lib/opcode.js
index 04c3ccf..c53bdc8 100644
--- a/lib/opcode.js
+++ b/lib/opcode.js
@@ -28,7 +28,10 @@ Opcode.prototype.toNumber = function() {
 };
 
 Opcode.prototype.fromString = function(str) {
-  this.num = Opcode.map[str];
+  var num = Opcode.map[str];
+  if (typeof num === 'undefined')
+    throw new Error('Invalid opcodestr');
+  this.num = num;
   return this;
 };