diff --git a/browser.js b/browser.js
index e223c63..66806d5 100644
--- a/browser.js
+++ b/browser.js
@@ -100,6 +100,8 @@ exports.verify = function(publicKey, msg, sig) {
 
 var derive = exports.derive = function(privateKeyA, publicKeyB) {
   return new Promise(function(resolve) {
+    assert(Buffer.isBuffer(privateKeyA), "Bad input");
+    assert(Buffer.isBuffer(publicKeyB), "Bad input");
     assert(privateKeyA.length === 32, "Bad private key");
     assert(publicKeyB.length === 65, "Bad public key");
     assert(publicKeyB[0] === 4, "Bad public key");
diff --git a/ecdh.cc b/ecdh.cc
index 840f8ca..9a21024 100644
--- a/ecdh.cc
+++ b/ecdh.cc
@@ -70,15 +70,15 @@ NAN_METHOD(Derive) {
   NanScope();
 
   if (args.Length() != 2 ||
-      !args[0]->IsObject() ||  // privkey_a
-      !args[1]->IsObject()) {  // pubkey_b
+      !node::Buffer::HasInstance(args[0]) ||  // privkey_a
+      !node::Buffer::HasInstance(args[1])) {  // pubkey_b
     return NanThrowError("Bad input");
   }
 
-  char* privkey_a = node::Buffer::Data(args[0]->ToObject());
-  size_t privkey_a_len = node::Buffer::Length(args[0]->ToObject());
-  char* pubkey_b = node::Buffer::Data(args[1]->ToObject());
-  size_t pubkey_b_len = node::Buffer::Length(args[1]->ToObject());
+  char* privkey_a = node::Buffer::Data(args[0]);
+  size_t privkey_a_len = node::Buffer::Length(args[0]);
+  char* pubkey_b = node::Buffer::Data(args[1]);
+  size_t pubkey_b_len = node::Buffer::Length(args[1]);
   if (privkey_a == NULL ||
       privkey_a_len != PRIVKEY_SIZE ||
       pubkey_b == NULL ||
diff --git a/test.js b/test.js
index d982616..93c3e77 100644
--- a/test.js
+++ b/test.js
@@ -142,6 +142,13 @@ describe("ECDH", function() {
       });
     });
   });
+
+  it("should reject promise on bad arguments", function(done) {
+    eccrypto.derive({}, {}).catch(function(e) {
+      expect(e.message).to.match(/bad input/i);
+      done();
+    });
+  });
 });
 
 describe("ECIES", function() {