1 files changed, 2 insertions, 2 deletions

diff --git a/src/openssl.c b/src/openssl.c
index 22c6e85..d8eebb5 100644
--- a/src/openssl.c
+++ b/src/openssl.c
@@ -1861,13 +1861,13 @@ static BIGNUM *(checkbig)(lua_State *L, int index, _Bool *lvalue) {
                 if (hex) {
                         luaL_argcheck(L, len > 2+(size_t)neg, index, "invalid hex string");
                         for (i = 2+neg; i < len; i++) {
-                                if (!isxdigit(str[i]))
+                                if (!isxdigit((unsigned char)str[i]))
                                         luaL_argerror(L, 1, "invalid hex string");
                         }
                 } else {
                         luaL_argcheck(L, len > neg, index, "invalid decimal string");
                         for (i = neg; i < len; i++) {
-                                if (!isdigit(str[i]))
+                                if (!isdigit((unsigned char)str[i]))
                                         luaL_argerror(L, 1, "invalid decimal string");
                         }
                 }