diff --git a/util/async.lua b/util/async.lua
index c81f8639b..d0bc6c3d6 100644
--- a/util/async.lua
+++ b/util/async.lua
@@ -37,6 +37,8 @@ local function waiter(num)
 		num = num - 1;
 		if num == 0 and waiting then
 			runner_continue(thread);
+		elseif num < 0 then
+			error("done() called too many times");
 		end
 	end;
 end