diff --git a/lib/lambda/c3-validate-entry/card/validation.js b/lib/lambda/c3-validate-entry/card/validation.js
index 63acb4dd..886947be 100644
--- a/lib/lambda/c3-validate-entry/card/validation.js
+++ b/lib/lambda/c3-validate-entry/card/validation.js
@@ -32,6 +32,9 @@ export function validateCardNumber(cardNumber) {
 			const discover = new Discover(cardNumber);
 			return discover.validate();
 		default:
+			if (cardNumber.length < 13 || cardNumber.length > 19) {
+				return 'The card number must be between 13 and 19 digits long.';
+			}
 			return null; // Unknown card network; assume it's valid.
 	}
 }