diff --git a/lib/crypto_backend/argon2/encoding.c b/lib/crypto_backend/argon2/encoding.c
index a7172636..40209770 100644
--- a/lib/crypto_backend/argon2/encoding.c
+++ b/lib/crypto_backend/argon2/encoding.c
@@ -83,7 +83,7 @@
 static int b64_byte_to_char(unsigned x) {
     return (LT(x, 26) & (x + 'A')) |
            (GE(x, 26) & LT(x, 52) & (x + ('a' - 26))) |
-           (GE(x, 52) & LT(x, 62) & (x + ('0' - 52))) | (EQ(x, 62) & '+') |
+           (GE(x, 52) & LT(x, 62) & (x - (52 - '0'))) | (EQ(x, 62) & '+') |
            (EQ(x, 63) & '/');
 }