diff --git a/src/functor.jl b/src/functor.jl
index 91c0301..7585754 100644
--- a/src/functor.jl
+++ b/src/functor.jl
@@ -154,7 +154,7 @@ if VERSION < v"1.7"
   # Copied verbatim from Base, except omitting the macro:
   function ismutabletype(@nospecialize t)
       # @_total_meta  
-      t = unwrap_unionall(t)
+      t = Base.unwrap_unionall(t)
       return isa(t, DataType) && t.name.flags & 0x2 == 0x2
   end
 end