diff --git a/lib/reline.rb b/lib/reline.rb
index 339b8bc8d7..4c26ac7266 100644
--- a/lib/reline.rb
+++ b/lib/reline.rb
@@ -36,7 +36,7 @@ module Reline
     attr_accessor :config
     attr_accessor :key_stroke
     attr_accessor :line_editor
-    attr_accessor :ambiguous_width
+    attr_writer :ambiguous_width
     attr_accessor :last_incremental_search
     attr_reader :output
 
@@ -356,9 +356,14 @@ module Reline
       end
     end
 
+    def ambiguous_width
+      may_req_ambiguous_char_width unless defined? @ambiguous_width
+      @ambiguous_width
+    end
+
     private def may_req_ambiguous_char_width
       @ambiguous_width = 2 if Reline::IOGate == Reline::GeneralIO or STDOUT.is_a?(File)
-      return if ambiguous_width
+      return if @ambiguous_width
       Reline::IOGate.move_cursor_column(0)
       begin
         output.write "\u{25bd}"
diff --git a/test/reline/test_unicode.rb b/test/reline/test_unicode.rb
index fa7aad39c4..8b28cf1cee 100644
--- a/test/reline/test_unicode.rb
+++ b/test/reline/test_unicode.rb
@@ -9,4 +9,8 @@ class Reline::Unicode::Test < Reline::TestCase
   def test_get_mbchar_width
     assert_equal Reline.ambiguous_width, Reline::Unicode.get_mbchar_width('é')
   end
+
+  def test_ambiguous_width
+    assert_equal 2, Reline::Unicode.calculate_width('√', true)
+  end
 end