diff --git a/main.mjs b/main.mjs
index 8334f27..7ecc6c7 100644
--- a/main.mjs
+++ b/main.mjs
@@ -39,7 +39,9 @@ const grades = [
  */
 
 function getGrade(mark){
-	if (typeof(mark) != 'number')
+
+	if (isNaN(Number(mark))) return 'Ab'
+
 	for (let i=0;i<=grades.length-1;i++){
 		const {max} = grades[i]
 		printdebug({i,max})