diff --git a/collects/db/private/mysql/connection.rkt b/collects/db/private/mysql/connection.rkt
index d0c6488d0b..172ef2c887 100644
--- a/collects/db/private/mysql/connection.rkt
+++ b/collects/db/private/mysql/connection.rkt
@@ -597,9 +597,9 @@
       (cond [(<= (char->integer #\0) c (char->integer #\9))
              (- c (char->integer #\0))]
             [(<= (char->integer #\a) c (char->integer #\f))
-             (- c (char->integer #\a))]
+             (+ 10 (- c (char->integer #\a)))]
             [(<= (char->integer #\A) c (char->integer #\F))
-             (- c (char->integer #\A))])))
+             (+ 10 (- c (char->integer #\A)))])))
   (unless (and (string? s) (even? (string-length s))
                (regexp-match? #rx"[0-9a-zA-Z]*" s))
     (raise-type-error 'hex-string->bytes