diff --git a/src/main/resources/data/computercraft/lua/bios.lua b/src/main/resources/data/computercraft/lua/bios.lua
index c52fb2072..d31e20335 100644
--- a/src/main/resources/data/computercraft/lua/bios.lua
+++ b/src/main/resources/data/computercraft/lua/bios.lua
@@ -681,6 +681,7 @@ if http then
     local nativeCheckURL = http.checkURL
     http.checkURLAsync = nativeCheckURL
     http.checkURL = function(_url)
+        expect(1, _url, "string")
         local ok, err = nativeCheckURL(_url)
         if not ok then return ok, err end