diff --git a/src/compiler.jl b/src/compiler.jl
index 4cd4cc899..427ba9012 100644
--- a/src/compiler.jl
+++ b/src/compiler.jl
@@ -262,6 +262,7 @@ function build_model_info(input_expr)
         MacroTools.@match arg begin
             (::Type{T_}) | (name_::Type{T_}) => T
             name_::T_ => name
+            ::T_ => throw(ArgumentError("unnamed arguments are not supported"))
             x_ => x
         end
     end