diff --git a/Michelf/MarkdownExtra.php b/Michelf/MarkdownExtra.php
index 060a4ae..4f79ff7 100644
--- a/Michelf/MarkdownExtra.php
+++ b/Michelf/MarkdownExtra.php
@@ -229,7 +229,7 @@ protected function doExtraAttributes($tag_name, $attr, $defaultIdValue = null, $
 		}
 
 		// Split on components
-		preg_match_all('/[#.a-z][-_:a-zA-Z0-9=]+/', $attr, $matches);
+		preg_match_all('/[#.a-z][-_:a-zA-Z0-9=]+/', $attr ?? '', $matches);
 		$elements = $matches[0];
 
 		// Handle classes and IDs (only first ID taken into account)