diff --git a/lib/pathname_builtin.rb b/lib/pathname_builtin.rb
index 3d78619..d9c5d5b 100644
--- a/lib/pathname_builtin.rb
+++ b/lib/pathname_builtin.rb
@@ -195,14 +195,12 @@ class Pathname
 
   # :stopdoc:
 
-  SAME_PATHS = if File::FNM_SYSCASE.nonzero?
+  if File::FNM_SYSCASE.nonzero?
     # Avoid #zero? here because #casecmp can return nil.
-    proc {|a, b| a.casecmp(b) == 0}
+    private def same_paths?(a, b) a.casecmp(b) == 0 end
   else
-    proc {|a, b| a == b}
+    private def same_paths?(a, b) a == b end
   end
-  SAME_PATHS.freeze
-  private_constant :SAME_PATHS
 
   attr_reader :path
   protected :path
@@ -836,12 +834,12 @@ def relative_path_from(base_directory)
       base_prefix, basename = r
       base_names.unshift basename if basename != '.'
     end
-    unless SAME_PATHS[dest_prefix, base_prefix]
+    unless same_paths?(dest_prefix, base_prefix)
       raise ArgumentError, "different prefix: #{dest_prefix.inspect} and #{base_directory.inspect}"
     end
     while !dest_names.empty? &&
           !base_names.empty? &&
-          SAME_PATHS[dest_names.first, base_names.first]
+          same_paths?(dest_names.first, base_names.first)
       dest_names.shift
       base_names.shift
     end
diff --git a/test/pathname/test_ractor.rb b/test/pathname/test_ractor.rb
index f06b750..737e4a4 100644
--- a/test/pathname/test_ractor.rb
+++ b/test/pathname/test_ractor.rb
@@ -20,6 +20,11 @@ class Ractor
         x.join(Pathname("b"), Pathname("c"))
       end
       assert_equal(Pathname("a/b/c"), r.value)
+
+      r = Ractor.new Pathname("a") do |a|
+        Pathname("b").relative_path_from(a)
+      end
+      assert_equal(Pathname("../b"), r.value)
     end;
   end
 end