diff --git a/src/args.jl b/src/args.jl
index 3b6e0339..e4f137af 100644
--- a/src/args.jl
+++ b/src/args.jl
@@ -656,6 +656,7 @@ function default(k::Symbol)
         letter, key = axis_k
         return _axis_defaults_byletter[letter][key]
     end
+    k == :letter && return k # for type recipe processing
     k in _suppress_warnings || error("Unknown key: ", k)
 end