diff --git a/backend/mailmark.c b/backend/mailmark.c
index 2d805664..25a91ecb 100644
--- a/backend/mailmark.c
+++ b/backend/mailmark.c
@@ -138,7 +138,7 @@ int mailmark(struct zint_symbol *symbol, unsigned char source[], int length) {
     short int b[112];
     short int temp[112];
     short int cdv[112];
-    unsigned char data[25];
+    unsigned char data[26];
     int data_top, data_step;
     unsigned char check[7];
     short int extender[27];