diff --git a/doc/api/stream.md b/doc/api/stream.md
index 546aee482afd53..51da5945284b87 100644
--- a/doc/api/stream.md
+++ b/doc/api/stream.md
@@ -3084,10 +3084,17 @@ changes:
 -->
 
 * `stream` {Readable|Duplex|ReadableStream}
-* Returns: {boolean}
+* Returns: {boolean|null} - Only returns `null` if `stream` is not a valid `Readable`, `Duplex` or `ReadableStream`.
 
 Returns whether the stream is readable.
 
+### `stream.isWritable(stream)`
+
+* `stream` {Writable|Duplex|WritableStream}
+* Returns: {boolean|null} - Only returns `null` if `stream` is not a valid `Writable`, `Duplex` or `WritableStream`.
+
+Returns whether the stream is writable.
+
 ### `stream.Readable.from(iterable[, options])`
 
 <!-- YAML