diff --git a/control/Session.php b/control/Session.php
index a4f13b55c..e97d3ae9b 100644
--- a/control/Session.php
+++ b/control/Session.php
@@ -295,13 +295,20 @@ class Session {
 			// We still want to do this even if we have strict path checking for legacy code
 			$var = &$this->data;
 			$diffVar = &$this->changedData;
-			
+
+			// Iterate twice over the names - once to see if the value needs to be changed,
+			// and secondly to get the changed data value. This is done to solve a problem
+			// where iterating over the diff var would create empty arrays, and the value
+			// would then not be set, inadvertently clearing session values.
 			foreach($names as $n) {
 				$var = &$var[$n];
-				$diffVar = &$diffVar[$n];
 			}
 
 			if($var !== $val) {
+				foreach($names as $n) {
+					$diffVar = &$diffVar[$n];
+				}
+
 				$var = $val;
 				$diffVar = $val;
 			}
diff --git a/tests/control/SessionTest.php b/tests/control/SessionTest.php
index 78debff69..31e01782b 100644
--- a/tests/control/SessionTest.php
+++ b/tests/control/SessionTest.php
@@ -45,6 +45,13 @@ class SessionTest extends SapphireTest {
 		$this->assertEquals($session, array('Test' => 'Test', 'Test-2' => 'Test-2'));
 	}
 
+	function testSettingExistingDoesntClear() {
+		$s = new Session(array('something' => array('does' => 'exist')));
+
+		$s->inst_set('something.does', 'exist');
+		$this->assertEquals(array(), $s->inst_changedData());
+	}
+
 	/**
 	 * Check that changedData isn't populated with junk when clearing non-existent entries.
 	 */