diff --git a/src/Request.php b/src/Request.php
index fc4967f..f9db93c 100644
--- a/src/Request.php
+++ b/src/Request.php
@@ -99,6 +99,13 @@ class Request
 	 */
 	private $encoding = Request::ENCODING_QUERY;
 
+	/**
+	 * Cache of encoded data.
+	 *
+	 * @var string|null
+	 */
+	private $encodedData = null;
+
 	/**
 	 * @param cURL $curl
 	 */
@@ -120,7 +127,7 @@ public function setMethod($method)
 			throw new \InvalidArgumentException("Method [$method] not a valid HTTP method.");
 		}
 
-		if ($this->data && !static::$methods[$method]) {
+		if ($this->data && !$this->allowsData()) {
 			throw new \LogicException('Request has POST data, but tried changing HTTP method to one that does not allow POST data');
 		}
 
@@ -314,6 +321,16 @@ public function formatHeaders()
 		return $headers;
 	}
 
+	/**
+	 * Detemine if a request allows data to be set.
+	 *
+	 * @return bool
+	 */
+	public function allowsData()
+	{
+		return static::$methods[$this->method];
+	}
+
 	/**
 	 * Set the POST data to be sent with the request.
 	 *
@@ -321,11 +338,12 @@ public function formatHeaders()
 	 */
 	public function setData($data)
 	{
-		if ($data && !static::$methods[$this->method]) {
+		if ($data !== null && !$this->allowsData()) {
 			throw new \InvalidArgumentException("HTTP method [$this->method] does not allow POST data.");
 		}
 
 		$this->data = $data;
+		$this->encodedData = null;
 
 		return $this;
 	}
@@ -337,7 +355,7 @@ public function setData($data)
 	 */
 	public function hasData()
 	{
-		return (bool) $this->data;
+		return (bool) $this->getEncodedData();
 	}
 
 	/**
@@ -372,6 +390,7 @@ public function setEncoding($encoding)
 		}
 
 		$this->encoding = $encoding;
+		$this->encodedData = null;
 
 		return $this;
 	}
@@ -393,6 +412,10 @@ public function getEncoding()
 	 */
 	public function encodeData()
 	{
+		if ($this->data === null) {
+			return '';
+		}
+
 		switch ($this->encoding) {
 			case static::ENCODING_JSON:
 				return json_encode($this->data);
@@ -405,6 +428,19 @@ public function encodeData()
 		}
 	}
 
+	/**
+	 * Get the encoded POST data as a string.
+	 *
+	 * @return string
+	 */
+	public function getEncodedData()
+	{
+		if ($this->encodedData === null) {
+			$this->encodedData = $this->encodeData();
+		}
+		return $this->encodedData;
+	}
+
 	/**
 	 * Set a specific curl option for the request.
 	 *
diff --git a/src/cURL.php b/src/cURL.php
index ae72c41..24c0b7e 100644
--- a/src/cURL.php
+++ b/src/cURL.php
@@ -185,7 +185,7 @@ public function buildUrl($url, array $query)
 	 *
 	 * @return Request
 	 */
-	public function newRequest($method, $url, $data = array(), $encoding = Request::ENCODING_QUERY)
+	public function newRequest($method, $url, $data = null, $encoding = Request::ENCODING_QUERY)
 	{
 		$class = $this->requestClass;
 		$request = new $class($this);
@@ -264,7 +264,7 @@ public function prepareRequest(Request $request)
 		curl_setopt($this->ch, CURLOPT_HTTPHEADER, $request->formatHeaders());
 
 		if ($request->hasData()) {
-			curl_setopt($this->ch, CURLOPT_POSTFIELDS, $request->encodeData());
+			curl_setopt($this->ch, CURLOPT_POSTFIELDS, $request->getEncodedData());
 		}
 
 		if ($method === 'head') {
diff --git a/tests/unit/RequestTest.php b/tests/unit/RequestTest.php
index b477e0e..95aee91 100644
--- a/tests/unit/RequestTest.php
+++ b/tests/unit/RequestTest.php
@@ -50,6 +50,50 @@ public function encodeData()
 		$this->assertEquals('<rawData>ArbitraryValue</rawData>', $r->encodeData());
 	}
 
+	/** @test */
+	public function encodeJsonData()
+	{
+		$r = $this->makeRequest();
+		$r->setMethod('post');
+		$r->setEncoding(Request::ENCODING_JSON);
+
+		$r->setData([]);
+		$this->assertEquals('[]', $r->encodeData());
+
+		$r->setData(new \stdClass);
+		$this->assertEquals('{}', $r->encodeData());
+	}
+
+	/** @test */
+	public function changeDataUpdatesEncodedData()
+	{
+		$r = $this->makeRequest();
+		$r->setMethod('post');
+		$r->setEncoding(Request::ENCODING_JSON);
+
+		$r->setData(['foo' => 'bar']);
+		$data1 = $r->getEncodedData();
+		$r->setData(['bar' => 'qux']);
+		$data2 = $r->getEncodedData();
+
+		$this->assertNotEquals($data1, $data2);
+	}
+
+	/** @test */
+	public function changeEncodingUpdatesEncodedData()
+	{
+		$r = $this->makeRequest();
+		$r->setMethod('post');
+		$r->setData(['foo' => 'bar']);
+
+		$r->setEncoding(Request::ENCODING_JSON);
+		$jsonData = $r->getEncodedData();
+		$r->setEncoding(Request::ENCODING_QUERY);
+		$queryData = $r->getEncodedData();
+
+		$this->assertNotEquals($jsonData, $queryData);
+	}
+
 	/** @test */
 	public function formatHeaders()
 	{