diff --git a/tiny_obj_loader.h b/tiny_obj_loader.h
index 4a43498..826ec24 100644
--- a/tiny_obj_loader.h
+++ b/tiny_obj_loader.h
@@ -175,8 +175,8 @@ MaterialReader::~MaterialReader() {}
 
 struct vertex_index {
   int v_idx, vt_idx, vn_idx;
-  vertex_index() {}
-  vertex_index(int idx) : v_idx(idx), vt_idx(idx), vn_idx(idx) {}
+  vertex_index() : v_idx(-1), vt_idx(-1), vn_idx(-1) {}
+  explicit vertex_index(int idx) : v_idx(idx), vt_idx(idx), vn_idx(idx) {}
   vertex_index(int vidx, int vtidx, int vnidx)
       : v_idx(vidx), vt_idx(vtidx), vn_idx(vnidx) {}
 };