diff --git a/lib/psych/visitors/yaml_tree.rb b/lib/psych/visitors/yaml_tree.rb
index 4abdda31..318f5f89 100644
--- a/lib/psych/visitors/yaml_tree.rb
+++ b/lib/psych/visitors/yaml_tree.rb
@@ -267,7 +267,7 @@ def visit_String o
           tag   = 'tag:yaml.org,2002:str'
           plain = false
           quote = false
-        elsif o == 'y' || o == 'n'
+        elsif o == 'y' || o == 'Y' || o == 'n' || o == 'N'
           style = Nodes::Scalar::DOUBLE_QUOTED
         elsif @line_width && o.length > @line_width
           style = Nodes::Scalar::FOLDED
diff --git a/test/psych/test_string.rb b/test/psych/test_string.rb
index 0dc34b30..84ae5cbb 100644
--- a/test/psych/test_string.rb
+++ b/test/psych/test_string.rb
@@ -17,17 +17,17 @@ def initialize
       end
     end
 
-    # 'y' and 'n' are kind of ambiguous.  Syck treated y and n literals in
+    # 'y', 'Y', 'n', 'N' are kind of ambiguous.  Syck treated those literals in
     # YAML documents as strings.  But this is not what the YAML 1.1 spec says.
     # YAML 1.1 says they should be treated as booleans.  When we're dumping
     # documents, we know it's a string, so adding quotes will eliminate the
     # "ambiguity" in the emitted document
-    def test_y_is_quoted
-      assert_match(/"y"/, Psych.dump("y"))
-    end
 
-    def test_n_is_quoted
-      assert_match(/"n"/, Psych.dump("n"))
+    def test_all_yaml_1_1_booleans_are_quoted
+      yaml_1_1_booleans = %w[y Y yes Yes YES n N no No NO true True TRUE false False FALSE on On ON off Off OFF] # from https://yaml.org/type/bool.html
+      yaml_1_1_booleans.each do |boolean|
+        assert_match(/"#{boolean}"|'#{boolean}'/, Psych.dump(boolean))
+      end
     end
 
     def test_string_with_newline