diff --git a/src/testing/exercise.md b/src/testing/exercise.md
index c255c770..b50f6d3d 100644
--- a/src/testing/exercise.md
+++ b/src/testing/exercise.md
@@ -8,7 +8,8 @@ The [Luhn algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm) is used to
 validate credit card numbers. The algorithm takes a string as input and does the
 following to validate the credit card number:
 
-- Ignore all spaces. Reject numbers with fewer than two digits.
+- Ignore all spaces. Reject numbers with fewer than two digits. Reject letters
+  and other non-digit characters.
 
 - Moving from **right to left**, double every second digit: for the number
   `1234`, we double `3` and `1`. For the number `98765`, we double `6` and `8`.