diff --git a/includes/validation/class-amp-validation-manager.php b/includes/validation/class-amp-validation-manager.php
index d4c38aaeaab..7d2530a4a8a 100644
--- a/includes/validation/class-amp-validation-manager.php
+++ b/includes/validation/class-amp-validation-manager.php
@@ -1426,31 +1426,21 @@ public static function decorate_filter_source( $value ) {
 	 *     @type string $type Source type (core, plugin, mu-plugin, or theme).
 	 *     @type string $name Source name.
 	 *     @type string $function Normalized function name.
-	 *     @type ReflectionMethod|ReflectionFunction $reflection
+	 *     @type ReflectionMethod|ReflectionFunction $reflection Reflection.
 	 * }
 	 */
 	public static function get_source( $callback ) {
 		$reflection = null;
-		$class_name = null; // Because ReflectionMethod::getDeclaringClass() can return a parent class.
 		try {
 			if ( is_string( $callback ) && is_callable( $callback ) ) {
 				// The $callback is a function or static method.
 				$exploded_callback = explode( '::', $callback, 2 );
 				if ( 2 === count( $exploded_callback ) ) {
-					$class_name = $exploded_callback[0];
 					$reflection = new ReflectionMethod( $exploded_callback[0], $exploded_callback[1] );
 				} else {
 					$reflection = new ReflectionFunction( $callback );
 				}
 			} elseif ( is_array( $callback ) && isset( $callback[0], $callback[1] ) && method_exists( $callback[0], $callback[1] ) ) {
-				// The $callback is a method.
-				if ( is_string( $callback[0] ) ) {
-					$class_name = $callback[0];
-				} elseif ( is_object( $callback[0] ) ) {
-					$class_name = get_class( $callback[0] );
-				}
-
-				// This is needed later for AMP_Validation_Manager::has_parameters_passed_by_reference().
 				$reflection = new ReflectionMethod( $callback[0], $callback[1] );
 
 				// Handle the special case of the class being a widget, in which case the display_callback method should
@@ -1465,6 +1455,7 @@ public static function get_source( $callback ) {
 			return null;
 		}
 
+		// The reflection is needed later for AMP_Validation_Manager::has_parameters_passed_by_reference().
 		if ( ! $reflection ) {
 			return null;
 		}
@@ -1501,8 +1492,8 @@ public static function get_source( $callback ) {
 			$source['line'] = $reflection->getStartLine();
 		}
 
-		if ( $class_name ) {
-			$source['function'] = $class_name . '::' . $reflection->getName();
+		if ( $reflection instanceof ReflectionMethod ) {
+			$source['function'] = $reflection->getDeclaringClass()->getName() . '::' . $reflection->getName();
 		} else {
 			$source['function'] = $reflection->getName();
 		}