diff --git a/lib/opcode.js b/lib/opcode.js
index 80af9f9..c48bbc1 100644
--- a/lib/opcode.js
+++ b/lib/opcode.js
@@ -1,6 +1,7 @@
 'use strict';
 
 var _ = require('lodash');
+var $ = require('./util/preconditions');
 
 function Opcode(num) {
   if (!(this instanceof Opcode)) {
@@ -19,11 +20,13 @@ function Opcode(num) {
 }
 
 Opcode.prototype.set = function(obj) {
+  $.checkArgument(_.isObject(obj));
   this.num = typeof obj.num !== 'undefined' ? obj.num : this.num;
   return this;
 };
 
 Opcode.prototype.fromNumber = function(num) {
+  $.checkArgument(_.isNumber(num));
   this.num = num;
   return this;
 };
@@ -33,9 +36,11 @@ Opcode.prototype.toNumber = function() {
 };
 
 Opcode.prototype.fromString = function(str) {
+  $.checkArgument(_.isString(str));
   var num = Opcode.map[str];
-  if (typeof num === 'undefined')
+  if (typeof num === 'undefined') {
     throw new Error('Invalid opcodestr');
+  }
   this.num = num;
   return this;
 };
@@ -49,9 +54,7 @@ Opcode.prototype.toString = function() {
 };
 
 Opcode.smallInt = function(n) {
-  if (!(n >= 0 && n <= 16)) {
-    throw new Error('Invalid Argument: n must be between 0 and 16');
-  }
+  $.checkArgument(n >= 0 && n <= 16, 'Invalid Argument: n must be between 0 and 16');
   if (n === 0) {
     return Opcode('OP_0');
   }
diff --git a/test/opcode.js b/test/opcode.js
index 41ccc43..9d06b69 100644
--- a/test/opcode.js
+++ b/test/opcode.js
@@ -25,52 +25,61 @@ describe('Opcode', function() {
   });
 
   describe('#fromNumber', function() {
-
     it('should work for 0', function() {
       Opcode().fromNumber(0).num.should.equal(0);
     });
+    it('should fail for non-number', function() {
+      Opcode().fromNumber.bind(null, 'a string').should.throw('Invalid Argument');
+    });
+  });
 
+  describe('#set', function() {
+    it('should work for object', function() {
+      Opcode().set({
+        num: 42
+      }).num.should.equal(42);
+    });
+    it('should fail for non-object', function() {
+      Opcode().set.bind(null, 'non-object').should.throw('Invalid Argument');
+    });
   });
 
   describe('#toNumber', function() {
-
     it('should work for 0', function() {
       Opcode().fromNumber(0).toNumber().should.equal(0);
     });
-
   });
 
   describe('#fromString', function() {
-
     it('should work for OP_0', function() {
       Opcode().fromString('OP_0').num.should.equal(0);
     });
-
+    it('should fail for invalid string', function() {
+      Opcode().fromString.bind(null, 'OP_SATOSHI').should.throw('Invalid opcodestr');
+      Opcode().fromString.bind(null, 'BANANA').should.throw('Invalid opcodestr');
+    });
+    it('should fail for non-string', function() {
+      Opcode().fromString.bind(null, 123).should.throw('Invalid Argument');
+    });
   });
 
   describe('#toString', function() {
-
     it('should work for OP_0', function() {
       Opcode().fromString('OP_0').toString().should.equal('OP_0');
     });
-
   });
 
   describe('@map', function() {
-
     it('should have a map containing 116 elements', function() {
       _.size(Opcode.map).should.equal(116);
     });
-
   });
 
   describe('@reverseMap', function() {
-
     it('should exist and have op 185', function() {
       should.exist(Opcode.reverseMap);
       Opcode.reverseMap[185].should.equal('OP_NOP10');
     });
-
   });
   var smallints = [
     Opcode('OP_0'),