diff --git a/src/nf/nf_layer.f90 b/src/nf/nf_layer.f90
index 5bcac6b..487e112 100644
--- a/src/nf/nf_layer.f90
+++ b/src/nf/nf_layer.f90
@@ -154,7 +154,7 @@ module subroutine set_params(self, params)
         !! Parameters of this layer
     end subroutine set_params
 
-    module subroutine set_state(self, state)
+    pure module subroutine set_state(self, state)
       class(layer), intent(inout) :: self
       real, intent(in), optional :: state(:)
     end subroutine set_state