diff --git a/glfw_base.lua b/glfw_base.lua
index 27dac64..a6cd2b0 100644
--- a/glfw_base.lua
+++ b/glfw_base.lua
@@ -5,6 +5,8 @@ local jit = require "jit"
 
 local Lib = {}
 
+setmetatable(glc, {__index=function(self, k) error("Unknown GL constant: "..k) end})
+
 -- Load and export libraries
 local gl, glu, glfw
 if ffi.os == "Windows" then
diff --git a/test.lua b/test.lua
index c2b5070..3e03f97 100644
--- a/test.lua
+++ b/test.lua
@@ -6,6 +6,8 @@ local bit = require "bit"
 -- Localize the FFI libraries
 local gl, glc, glu, glfw, glext = lj_glfw.libraries()
 
+assert(pcall(function() local _ = glc.this_doesnt_exist end) == false)
+
 -- Define vertex arrays for the model we will draw
 local CubeVerticies = {}
 CubeVerticies.v = ffi.new("const float[8][3]", {