diff --git a/mrbgems/mruby-array-ext/src/array.c b/mrbgems/mruby-array-ext/src/array.c
index ddd4bc858..fddc65596 100644
--- a/mrbgems/mruby-array-ext/src/array.c
+++ b/mrbgems/mruby-array-ext/src/array.c
@@ -1360,6 +1360,7 @@ ary_product_group(mrb_state *mrb, mrb_value self_ary)
   while (j > 0) {
     j -= 1;
     mrb_value a = RARRAY_PTR(arys_ary)[j]; // arys[j]
+    mrb_check_type(mrb, a, MRB_TT_ARRAY);
     mrb_int b = RARRAY_LEN(a);             // a.size
     mrb_ary_set(mrb, group, j + 1, RARRAY_PTR(a)[n % b]);
     n /= b;