diff --git a/src/interface.jl b/src/interface.jl
index 8f7d700..9441c96 100644
--- a/src/interface.jl
+++ b/src/interface.jl
@@ -273,7 +273,7 @@ macro def(expr)
   # Positional-argument method, has defaults for all but the first arg:
   positional = :(function $rule($(names[1]), $(params[2:end]...))
     $check_sign_eta
-    vars = $(maybe_float).(($(names...)),($(default_types...)))
+    vars = $(maybe_float).([$(names...)],[$(default_types...)])
     return new{typeof.(vars)...}(vars...)
   end)
   # Keyword-argument method. (Made an inner constructor only to allow