diff --git a/src/stringify/stringifyString.ts b/src/stringify/stringifyString.ts
index 04a33945..9caab598 100644
--- a/src/stringify/stringifyString.ts
+++ b/src/stringify/stringifyString.ts
@@ -280,7 +280,7 @@ function plainString(
   const { type, value } = item
   const { actualString, implicitKey, indent, indentStep, inFlow } = ctx
   if (
-    (implicitKey && /[\n[\]{},]/.test(value)) ||
+    (implicitKey && value.includes('\n')) ||
     (inFlow && /[[\]{},]/.test(value))
   ) {
     return quotedString(value, ctx)
diff --git a/tests/doc/stringify.ts b/tests/doc/stringify.ts
index 5f68dfd4..a094203a 100644
--- a/tests/doc/stringify.ts
+++ b/tests/doc/stringify.ts
@@ -336,6 +336,11 @@ z:
     expect(String(doc)).toBe('a: 1\n? b: 2\n  c: 3\n')
   })
 
+  test('plain key containing flow indicators', () => {
+    const doc = YAML.parseDocument('foo[{}]: bar')
+    expect(doc.toString()).toBe('foo[{}]: bar\n')
+  })
+
   describe('No extra whitespace for empty values', () => {
     const getDoc = () =>
       new YAML.Document<YAML.YAMLMap<YAML.Scalar, YAML.Scalar>, false>({