diff --git a/anselme/ast/Struct.lua b/anselme/ast/Struct.lua
index b7da15b..51712fa 100644
--- a/anselme/ast/Struct.lua
+++ b/anselme/ast/Struct.lua
@@ -113,7 +113,7 @@ Struct = ast.abstract.Runtime {
 		if v ~= nil then
 			return v
 		else
-			return Nil:new()
+			error(("key %q is undefined in %s"):format(key:format(), self.type), 0)
 		end
 	end,
 	get_strict = function(self, key)
diff --git a/ideas.md b/ideas.md
index 38b26bd..d265bab 100644
--- a/ideas.md
+++ b/ideas.md
@@ -25,10 +25,6 @@ Issue: dispatch is decided before evaluating default values.
 
 Multiline string and comments?
 
----
-
-Error on undefined struct/table key? I copied the Lua behavior but maybe not useful here.
-
 # Can be done later
 
 Translation.