diff --git a/lib/bitcoind.js b/lib/bitcoind.js
index 71510727..85914ea0 100644
--- a/lib/bitcoind.js
+++ b/lib/bitcoind.js
@@ -317,6 +317,8 @@ function Block(data) {
 
   var self = this;
 
+  this._isBlock = Block._blockFlag;
+
   Object.keys(data).forEach(function(key) {
     if (!self[key]) {
       self[key] = data[key];
@@ -326,6 +328,12 @@ function Block(data) {
   this.toHex();
 }
 
+Block._blockFlag = {};
+
+Block.isBlock = function(block) {
+  return block._isBlock === Block._blockFlag;
+};
+
 Block.prototype.verify = function() {
   return this.verified = this.verified || bitcoindjs.verifyBlock(this);
 };
@@ -418,6 +426,8 @@ function Transaction(data) {
 
   var self = this;
 
+  this._isTx = Transaction._txFlag;
+
   this.nMinTxFee = data.nMinTxFee || data.minTxFee || 1000;
   this.nMinRelayTxFee = data.nMinRelayTxFee || data.minRelayTxFee || 1000;
   this.CURRENT_VERSION = 1;
@@ -444,25 +454,42 @@ function Transaction(data) {
   this.toHex();
 }
 
+Transaction._txFlag = {};
+
+Transaction.isTransaction =
+Transaction.isTx = function(tx) {
+  return tx._isTx === Transaction._txFlag;
+};
+
 Transaction.prototype.verify = function() {
   return this.verified = this.verified || bitcoindjs.verifyTransaction(this);
 };
 
 Transaction.prototype.sign =
 Transaction.prototype.fill = function() {
-  var self = this;
-  var tx = Transaction.fill(this);
-  Object.keys(tx).forEach(function(key) {
-    self[key] = tx[key];
-  });
-  self.toHex();
+  return Transaction.fill(this);
 };
 
 Transaction.sign =
 Transaction.fill = function(tx) {
-  tx = bitcoin.tx(tx);
-  tx = bitcoindjs.fillTransaction(tx);
-  return bitcoin.tx(tx);
+  var isTx = bitcoin.tx.isTx(tx)
+    , newTx;
+
+  if (!isTx) {
+    tx = bitcoin.tx(tx);
+  }
+
+  try {
+    newTx = bitcoindjs.fillTransaction(tx);
+  } catch (e) {
+    return false;
+  }
+
+  Object.keys(newTx).forEach(function(key) {
+    tx[key] = newTx[key];
+  });
+
+  return isTx ? true : tx;
 };
 
 Transaction.prototype.getSerializeSize = function() {