diff --git a/src/lib_io.c b/src/lib_io.c
index 037aa28e..a502dd52 100644
--- a/src/lib_io.c
+++ b/src/lib_io.c
@@ -278,6 +278,15 @@ static int io_file_iter(lua_State *L)
   return n;
 }
 
+static int io_file_lines(lua_State *L)
+{
+  int n = (int)(L->top - L->base);
+  if (n > LJ_MAX_UPVAL)
+    lj_err_caller(L, LJ_ERR_UNPACK);
+  lua_pushcclosure(L, io_file_iter, n);
+  return 1;
+}
+
 /* -- I/O file methods ---------------------------------------------------- */
 
 #define LJLIB_MODULE_io_method
@@ -361,8 +370,7 @@ LJLIB_CF(io_method_setvbuf)
 LJLIB_CF(io_method_lines)
 {
   io_tofile(L);
-  lua_pushcclosure(L, io_file_iter, (int)(L->top - L->base));
-  return 1;
+  return io_file_lines(L);
 }
 
 LJLIB_CF(io_method___gc)
@@ -492,8 +500,7 @@ LJLIB_CF(io_lines)
   } else {  /* io.lines() iterates over stdin. */
     setudataV(L, L->base, IOSTDF_UD(L, GCROOT_IO_INPUT));
   }
-  lua_pushcclosure(L, io_file_iter, (int)(L->top - L->base));
-  return 1;
+  return io_file_lines(L);
 }
 
 LJLIB_CF(io_type)