diff --git a/pino.d.ts b/pino.d.ts
index 2759a1de9..033ae2cd1 100644
--- a/pino.d.ts
+++ b/pino.d.ts
@@ -326,7 +326,7 @@ declare namespace pino {
     type PlaceholderTypeMapping<T extends PlaceholderSpecifier> = T extends 'd'
         ? number
         : T extends 's'
-            ? string | number | boolean | bigint | null | undefined | symbol
+            ? unknown
             : T extends 'j' | 'o' | 'O'
             ? object
             : never;
@@ -878,4 +878,4 @@ declare function pino<CustomLevels extends string = never, UseOnlyCustomLevels e
  */
 declare function pino<CustomLevels extends string = never, UseOnlyCustomLevels extends boolean = boolean>(options: pino.LoggerOptions<CustomLevels, UseOnlyCustomLevels>, stream?: pino.DestinationStream | undefined): pino.Logger<CustomLevels, UseOnlyCustomLevels>;
 
-export = pino;
\ No newline at end of file
+export = pino;
diff --git a/test/types/pino.test-d.ts b/test/types/pino.test-d.ts
index bdb860a31..0e6df8f89 100644
--- a/test/types/pino.test-d.ts
+++ b/test/types/pino.test-d.ts
@@ -71,8 +71,9 @@ info({ a: 1, b: '2' }, 'hello world with %s', 'extra data');
 // Extra message after placeholder
 expectError(info({ a: 1, b: '2' }, 'hello world with %d', 2, 'extra' ));
 
-// metadata with messages type errors
-expectError(info({ a: 1, b: '2' }, 'hello world with %s', {}));
+// metadata with messages type passes, because of custom toString method
+// We can't detect if the object has a custom toString method that returns a string
+info({ a: 1, b: '2' }, 'hello world with %s', {});
 
 // metadata after message
 expectError(info('message', { a: 1, b: '2' }));