diff --git a/ext/pathname/lib/pathname.rb b/ext/pathname/lib/pathname.rb
index 7fb923c6ae..383d47b52f 100644
--- a/ext/pathname/lib/pathname.rb
+++ b/ext/pathname/lib/pathname.rb
@@ -503,9 +503,13 @@ class Pathname
   # ArgumentError is raised when it cannot find a relative path.
   #
   def relative_path_from(base_directory)
-    base_directory = Pathname.new(base_directory) unless Pathname === base_directory
     dest_directory = self.cleanpath.to_s
-    base_directory = base_directory.cleanpath.to_s
+    base_directory =
+      if base_directory.respond_to? :cleanpath
+        base_directory
+      else
+        Pathname.new(base_directory)
+      end.cleanpath.to_s
     dest_prefix = dest_directory
     dest_names = []
     while r = chop_basename(dest_prefix)
diff --git a/test/pathname/test_pathname.rb b/test/pathname/test_pathname.rb
index 3bea182ed4..fbf4bb3459 100644
--- a/test/pathname/test_pathname.rb
+++ b/test/pathname/test_pathname.rb
@@ -1429,4 +1429,18 @@ class TestPathname < Test::Unit::TestCase
       assert_instance_of(Pathname, foo.relative_path_from(bar))
     end;
   end
+
+  def test_relative_path_from_mock
+    assert_equal(
+      Pathname.new("../bar"),
+      Pathname.new("/foo/bar").relative_path_from(Pathname.new("/foo/baz")))
+    assert_equal(
+      Pathname.new("../bar"),
+      Pathname.new("/foo/bar").relative_path_from("/foo/baz"))
+    obj = Object.new
+    def obj.cleanpath() Pathname.new("/foo/baz") end
+    assert_equal(
+      Pathname.new("../bar"),
+      Pathname.new("/foo/bar").relative_path_from(obj))
+  end
 end