diff --git a/frontend/src/pages/QuizzesPage.jsx b/frontend/src/pages/QuizzesPage.jsx
index 13634ef..47d0cb6 100644
--- a/frontend/src/pages/QuizzesPage.jsx
+++ b/frontend/src/pages/QuizzesPage.jsx
@@ -229,7 +229,8 @@ function QuizCard({ quiz, isModerator, categories, onDelete, onCategoryChange })
 
   const assignCategory = async (catId) => {
     try {
-      await api.patch(`/categories/quizzes/${quiz.id}`, null, { params: { category_id: catId ?? '' } })
+      const params = catId == null ? {} : { category_id: catId }
+      await api.patch(`/categories/quizzes/${quiz.id}`, null, { params })
       onCategoryChange(quiz.id, catId)
     } catch { }
     setShowCatMenu(false)