diff --git a/exercises/practice/phone-number/.docs/instructions.md b/exercises/practice/phone-number/.docs/instructions.md
index 5d4d3739..806947e2 100644
--- a/exercises/practice/phone-number/.docs/instructions.md
+++ b/exercises/practice/phone-number/.docs/instructions.md
@@ -31,4 +31,6 @@ should all produce the output
 
 `6139950253`
 
+Any invalid phone number should produce a `null` output.
+
 **Note:** As this exercise only deals with telephone numbers used in NANP-countries, only 1 is considered a valid country code.