diff --git a/bn_mp_read_radix.c b/bn_mp_read_radix.c
index dc41065..18d301c 100644
--- a/bn_mp_read_radix.c
+++ b/bn_mp_read_radix.c
@@ -48,7 +48,7 @@ int mp_read_radix (mp_int * a, const char *str, int radix)
      * this allows numbers like 1AB and 1ab to represent the same  value
      * [e.g. in hex]
      */
-    ch = (char) ((radix < 36) ? toupper (*str) : *str);
+    ch = (char) ((radix < 36) ? toupper ((int)*str) : *str);
     for (y = 0; y < 64; y++) {
       if (ch == mp_s_rmap[y]) {
          break;
diff --git a/pre_gen/mpi.c b/pre_gen/mpi.c
index 07091a2..c0f860c 100644
--- a/pre_gen/mpi.c
+++ b/pre_gen/mpi.c
@@ -6416,7 +6416,7 @@ int mp_read_radix (mp_int * a, const char *str, int radix)
      * this allows numbers like 1AB and 1ab to represent the same  value
      * [e.g. in hex]
      */
-    ch = (char) ((radix < 36) ? toupper (*str) : *str);
+    ch = (char) ((radix < 36) ? toupper ((int)*str) : *str);
     for (y = 0; y < 64; y++) {
       if (ch == mp_s_rmap[y]) {
          break;