diff --git a/first/ackbuilder.lua b/first/ackbuilder.lua
index 1a150cb4a..0f83c4936 100644
--- a/first/ackbuilder.lua
+++ b/first/ackbuilder.lua
@@ -361,7 +361,8 @@ local function loadbuildfile(filename)
 			data, e = fp:read("*a")
 			fp:close()
 			if not e then
-				local thisglobals = {_G = thisglobals}
+				local thisglobals = {}
+				thisglobals._G = thisglobals
 				setmetatable(thisglobals, {__index = globals})
 				chunk, e = load(data, "@"..filename, "text", thisglobals)
 			end
@@ -787,6 +788,18 @@ setmetatable(globals,
 vars.cflags = {}
 parente.vars = vars
 
+setmetatable(_G,
+	{
+		__index = function(self, k)
+			local value = rawget(_G, k)
+			if not value then
+				error(string.format("access of undefined variable '%s'", k))
+			end
+			return value
+		end
+	}
+)
+		
 do
 	local emitter_type = install_make_emitter
 	parse_arguments(
diff --git a/lang/cem/cpp.ansi/build.lua b/lang/cem/cpp.ansi/build.lua
index 0cc57fe9f..1f22fe88f 100644
--- a/lang/cem/cpp.ansi/build.lua
+++ b/lang/cem/cpp.ansi/build.lua
@@ -1,6 +1,6 @@
 include("util/cmisc/build.lua")
 
-local allocd_header = definerule(null,
+local allocd_header = definerule(nil,
 	{
 		srcs = { type="targets" }
 	},
diff --git a/mach/proto/ncg/build.lua b/mach/proto/ncg/build.lua
index e093f891e..5245fd2a7 100644
--- a/mach/proto/ncg/build.lua
+++ b/mach/proto/ncg/build.lua
@@ -26,8 +26,7 @@ definerule("build_ncg",
 			srcs = {
 				"mach/proto/ncg/*.c",
 				"mach/"..e.arch.."/ncg/mach.h",
-				archlib, -- for .c file
-				tables, -- for .c file
+				matching(filenamesof(tables), "%.c$")
 			},
 			deps = {
 				"h+emheaders",