diff --git a/dev/TestRunner.php b/dev/TestRunner.php
index 4c257e6edb4..741cc9a5198 100644
--- a/dev/TestRunner.php
+++ b/dev/TestRunner.php
@@ -92,6 +92,10 @@ public static function use_test_manifest() {
 		Config::inst()->pushConfigStaticManifest(new SS_ConfigStaticManifest(
 			BASE_PATH, true, isset($_GET['flush'])
 		));
+		
+		// Invalidate classname spec since the test manifest will now pull out new subclasses for each internal class
+		// (e.g. Member will now have various subclasses of DataObjects that implement TestOnly)
+		DataObject::clear_classname_spec_cache();
 	}
 
 	public function init() {
diff --git a/model/DataObject.php b/model/DataObject.php
index fc5fb6bbe6a..31975b968a1 100644
--- a/model/DataObject.php
+++ b/model/DataObject.php
@@ -192,6 +192,14 @@ public static function set_validation_enabled($enable) {
 	 * @var [string] - class => ClassName field definition cache for self::database_fields
 	 */
 	private static $classname_spec_cache = array();
+	
+	/**
+	 * Clear all cached classname specs. It's necessary to clear all cached subclassed names
+	 * for any classes if a new class manifest is generated.
+	 */
+	public static function clear_classname_spec_cache() {
+		self::$classname_spec_cache = array();
+	}
 
 	/**
 	 * Return the complete map of fields on this object, including "Created", "LastEdited" and "ClassName".
@@ -202,7 +210,7 @@ public static function set_validation_enabled($enable) {
 	 */
 	public static function database_fields($class) {
 		if(get_parent_class($class) == 'DataObject') {
-			if(!isset(self::$classname_spec_cache[$class])) {
+			if(empty(self::$classname_spec_cache[$class])) {
 				$classNames = ClassInfo::subclassesFor($class);
 
 				$db = DB::getConn();