diff --git a/src/app/admin/matrix-management/detail/components/searchModal.tsx b/src/app/admin/matrix-management/detail/components/searchModal.tsx
index 81c2177..1857232 100644
--- a/src/app/admin/matrix-management/detail/components/searchModal.tsx
+++ b/src/app/admin/matrix-management/detail/components/searchModal.tsx
@@ -217,9 +217,11 @@ export default function SearchModal({
     setAddSuccess('')
     setAdding(true)
     try {
+      // If advanced is not checked, or if advanced is checked but parentId is not set (root selected), use rootUserId as parentUserId
+      const effectiveParentId = (!advanced || !parentId) ? rootUserId : parentId;
       const data = await addUserToMatrix({
         childUserId: selected.userId,
-        parentUserId: advanced ? parentId : undefined,
+        parentUserId: effectiveParentId,
         forceParentFallback: forceFallback,
         rootUserId,
         matrixId,