diff --git a/grammar b/grammar
index 5d495d9455e6a5aaa861522a686e0c2d0f2ad368..2b5ae47e900c70ed7d75d0d6ac5ee64fb6835468 100644
--- a/grammar
+++ b/grammar
@@ -4,7 +4,7 @@
 Amount ::= Digit+ ("-" | "/" | "to")? Digit*
 
 /* Digit: */
-Digit ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
+Digit ::= ("0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9")+
 
 /* Unit: unit of measure */
 /* recognises synonyms or shorthand */