diff --git a/lib/awful/spawn.lua b/lib/awful/spawn.lua
index 3aa432dc..09a0bd0f 100644
--- a/lib/awful/spawn.lua
+++ b/lib/awful/spawn.lua
@@ -123,7 +123,9 @@ function spawn.with_line_callback(cmd, callbacks)
             done_before = true
             return
         end
-        done_callback()
+        if done_callback then
+            done_callback()
+        end
     end
     if have_stdout then
         spawn.read_lines(Gio.UnixInputStream.new(stdout, true),
diff --git a/spawn.c b/spawn.c
index 7d8957af..659dd5f8 100644
--- a/spawn.c
+++ b/spawn.c
@@ -402,7 +402,7 @@ luaA_spawn(lua_State *L)
         return_stdout = luaA_checkboolean(L, 4);
     if(lua_gettop(L) >= 5)
         return_stderr = luaA_checkboolean(L, 5);
-    if(lua_gettop(L) >= 6)
+    if (!lua_isnoneornil(L, 6))
     {
         luaA_checkfunction(L, 6);
         flags |= G_SPAWN_DO_NOT_REAP_CHILD;