diff --git a/Project.toml b/Project.toml
index 55cf1b9..ec972e7 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "DerivableInterfaces"
 uuid = "6c5e35bf-e59e-4898-b73c-732dcc4ba65f"
 authors = ["ITensor developers <support@itensor.org> and contributors"]
-version = "0.3.15"
+version = "0.3.16"
 
 [deps]
 Adapt = "79e6a3ab-5dfb-504d-930d-738a2a938a0e"
diff --git a/src/abstractinterface.jl b/src/abstractinterface.jl
index 1cee893..8223c6d 100644
--- a/src/abstractinterface.jl
+++ b/src/abstractinterface.jl
@@ -2,25 +2,31 @@
 interface(x) = interface(typeof(x))
 # TODO: Define as `DefaultInterface()`.
 interface(::Type) = error("Interface unknown.")
-interface(x1, x_rest...) = combine_interfaces(x1, x_rest...)
+interface(x1, x_rest...) = combine_interfaces(interface(x1), interface.(x_rest)...)
+
+abstract type AbstractInterface end
+
+(interface::AbstractInterface)(f) = InterfaceFunction(interface, f)
 
 # Adapted from `Base.Broadcast.combine_styles`.
 # Get the combined interfaces of the input objects.
-function combine_interfaces(x1, x2, x_rest...)
-  return combine_interfaces(combine_interfaces(x1, x2), x_rest...)
+function combine_interfaces(
+  inter1::AbstractInterface, inter2::AbstractInterface, inter_rest::AbstractInterface...
+)
+  return combine_interfaces(combine_interface_rule(inter1, inter2), inter_rest...)
+end
+function combine_interfaces(inter1::AbstractInterface, inter2::AbstractInterface)
+  return combine_interface_rule(inter1, inter2)
 end
-combine_interfaces(x1, x2) = combine_interface_rule(interface(x1), interface(x2))
-combine_interfaces(x) = interface(x)
+combine_interfaces(inter::AbstractInterface) = inter
 
 # Rules for combining interfaces.
 function combine_interface_rule(
-  interface1::Interface, interface2::Interface
-) where {Interface}
-  return interface1
+  inter1::Interface, inter2::Interface
+) where {Interface<:AbstractInterface}
+  return inter1
 end
 # TODO: Define as `UnknownInterface()`.
-combine_interface_rule(interface1, interface2) = error("No rule for combining interfaces.")
-
-abstract type AbstractInterface end
-
-(interface::AbstractInterface)(f) = InterfaceFunction(interface, f)
+function combine_interface_rule(inter1::AbstractInterface, inter2::AbstractInterface)
+  return error("No rule for combining interfaces.")
+end
diff --git a/src/derive_macro.jl b/src/derive_macro.jl
index 0d17527..7f73d41 100644
--- a/src/derive_macro.jl
+++ b/src/derive_macro.jl
@@ -165,9 +165,7 @@ function derive_func_from_types(types::Expr, func::Expr)
     end
     return argname(i)
   end
-  interface = globalref_derive(
-    :(DerivableInterfaces.combine_interfaces($(active_argnames...)))
-  )
+  interface = globalref_derive(:(DerivableInterfaces.interface($(active_argnames...))))
   return derive_interface_func(interface, new_func)
 end