diff --git a/lib/hafriedlander/Peg/Compiler/PHPBuilder.php b/lib/hafriedlander/Peg/Compiler/PHPBuilder.php
index ddb26eb..5fa58da 100644
--- a/lib/hafriedlander/Peg/Compiler/PHPBuilder.php
+++ b/lib/hafriedlander/Peg/Compiler/PHPBuilder.php
@@ -50,7 +50,7 @@ function b(...$args) {
 		return $this;
 	}
 
-	function replace(array $replacements, array &$array = \null) {
+	function replace(array $replacements, ?array &$array = \null) {
 		if ($array === \null) {
 			unset($array);
 			$array =& $this->lines;