diff --git a/README.md b/README.md
index 27c58d81..0425f57b 100644
--- a/README.md
+++ b/README.md
@@ -41,8 +41,9 @@ fn main() -> Result<(), serde_yaml::Error> {
     map.insert("y".to_string(), 2.0);
 
     // Serialize it to a YAML string.
+    // y is quoted to avoid ambiguity in parsers that might read it as `true`.
     let yaml = serde_yaml::to_string(&map)?;
-    assert_eq!(yaml, "x: 1.0\ny: 2.0\n");
+    assert_eq!(yaml, "x: 1.0\n'y': 2.0\n");
 
     // Deserialize it back to a Rust type.
     let deserialized_map: BTreeMap<String, f64> = serde_yaml::from_str(&yaml)?;
@@ -75,7 +76,7 @@ fn main() -> Result<(), serde_yaml::Error> {
     let point = Point { x: 1.0, y: 2.0 };
 
     let yaml = serde_yaml::to_string(&point)?;
-    assert_eq!(yaml, "x: 1.0\ny: 2.0\n");
+    assert_eq!(yaml, "x: 1.0\n'y': 2.0\n");
 
     let deserialized_point: Point = serde_yaml::from_str(&yaml)?;
     assert_eq!(point, deserialized_point);
diff --git a/src/de.rs b/src/de.rs
index f0b467da..571ae235 100644
--- a/src/de.rs
+++ b/src/de.rs
@@ -1101,13 +1101,20 @@ pub(crate) fn digits_but_not_number(scalar: &str) -> bool {
 /// serialize it with quotes to be safe.
 /// This avoids the norway problem https://hitchdev.com/strictyaml/why/implicit-typing-removed/
 pub(crate) fn ambiguous_string(scalar: &str) -> bool {
-    parse_bool(scalar).is_some()
-        || parse_null(scalar.as_bytes()).is_some()
-        || scalar.len() == 0
-        || scalar.bytes().nth(0).unwrap().is_ascii_digit()
-        || scalar.starts_with('-')
-        || scalar.starts_with('.')
-        || scalar.starts_with("+")
+    let lower_scalar = scalar.to_lowercase();
+    parse_bool(&lower_scalar).is_some()
+        || parse_null(&lower_scalar.as_bytes()).is_some()
+        || lower_scalar.len() == 0
+        || lower_scalar.bytes().nth(0).unwrap().is_ascii_digit()
+        || lower_scalar.starts_with('-')
+        || lower_scalar.starts_with('.')
+        || lower_scalar.starts_with("+")
+        // Things that we don't parse as bool but could be parsed as bool by
+        // other YAML parsers.
+        || lower_scalar == "y"
+        || lower_scalar == "yes"
+        || lower_scalar == "n"
+        || lower_scalar == "no"
 }
 
 pub(crate) fn visit_int<'de, V>(visitor: V, v: &str) -> Result<Result<V::Value>, V>
diff --git a/src/lib.rs b/src/lib.rs
index f22cee9d..b1440bae 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -24,8 +24,9 @@
 //!     map.insert("y".to_string(), 2.0);
 //!
 //!     // Serialize it to a YAML string.
+//!     // 'y' is quoted to avoid ambiguity in parsers that might read it as `true`.
 //!     let yaml = serde_yaml::to_string(&map)?;
-//!     assert_eq!(yaml, "x: 1.0\ny: 2.0\n");
+//!     assert_eq!(yaml, "x: 1.0\n'y': 2.0\n");
 //!
 //!     // Deserialize it back to a Rust type.
 //!     let deserialized_map: BTreeMap<String, f64> = serde_yaml::from_str(&yaml)?;
@@ -55,7 +56,7 @@
 //!     let point = Point { x: 1.0, y: 2.0 };
 //!
 //!     let yaml = serde_yaml::to_string(&point)?;
-//!     assert_eq!(yaml, "x: 1.0\ny: 2.0\n");
+//!     assert_eq!(yaml, "x: 1.0\n'y': 2.0\n");
 //!
 //!     let deserialized_point: Point = serde_yaml::from_str(&yaml)?;
 //!     assert_eq!(point, deserialized_point);
diff --git a/tests/test_serde.rs b/tests/test_serde.rs
index 22651f08..40e93ba2 100644
--- a/tests/test_serde.rs
+++ b/tests/test_serde.rs
@@ -195,7 +195,7 @@ fn test_map() {
     thing.insert("y".to_owned(), 2);
     let yaml = indoc! {"
         x: 1
-        y: 2
+        'y': 2
     "};
     test_serde(&thing, yaml);
 }
@@ -238,7 +238,7 @@ fn test_basic_struct() {
     };
     let yaml = indoc! {r#"
         x: -4
-        y: "hi\tquoted"
+        'y': "hi\tquoted"
         z: true
     "#};
     test_serde(&thing, yaml);
@@ -335,6 +335,23 @@ fn test_strings_needing_quote() {
         string: ''
     "};
     test_serde(&thing3, yaml3);
+
+    let thing4 = Struct2 {
+        string: " ".to_owned(),
+    };
+    let yaml4 = indoc! {"
+        string: ' '
+    "};
+    test_serde(&thing4, yaml4);
+
+    // The literal norway problem https://hitchdev.com/strictyaml/why/implicit-typing-removed/
+    let thing5 = Struct2 {
+        string: "NO".to_owned(),
+    };
+    let yaml5 = indoc! {"
+        string: 'NO'
+    "};
+    test_serde(&thing5, yaml5);
 }
 
 #[test]