diff --git a/src/CallbackWrapper.php b/src/CallbackWrapper.php
index 6071cd0..67f9110 100644
--- a/src/CallbackWrapper.php
+++ b/src/CallbackWrapper.php
@@ -97,7 +97,7 @@ public function stream_open($path, $mode, $options, &$opened_path) {
 	public function stream_read($count) {
 		$result = parent::stream_read($count);
 		if (is_callable($this->readCallback)) {
-			call_user_func($this->readCallback, $result);
+			call_user_func($this->readCallback, strlen($result));
 		}
 		return $result;
 	}
diff --git a/tests/CallbackWrapperTest.php b/tests/CallbackWrapperTest.php
index e1cf84f..d4b3a5d 100644
--- a/tests/CallbackWrapperTest.php
+++ b/tests/CallbackWrapperTest.php
@@ -31,8 +31,10 @@ public function testWrapInvalidSource() {
 
 	public function testReadCallback() {
 		$called = false;
-		$callBack = function () use (&$called) {
+		$bytesRead = 0;
+		$callBack = function ($count) use (&$called, &$bytesRead) {
 			$called = true;
+			$bytesRead += $count;
 		};
 
 		$source = fopen('php://temp', 'r+');
@@ -42,6 +44,9 @@ public function testReadCallback() {
 		$wrapped = $this->wrapSource($source, $callBack);
 		$this->assertEquals('foo', fread($wrapped, 3));
 		$this->assertTrue($called);
+
+		$this->assertEquals('bar', fread($wrapped, 1000));
+		$this->assertEquals(6, $bytesRead);
 	}
 
 	public function testWriteCallback() {