diff --git a/src/util/amount.c b/src/util/amount.c
index e0664853a..33ba9a2f7 100644
--- a/src/util/amount.c
+++ b/src/util/amount.c
@@ -52,7 +52,7 @@ TALER_string_to_amount (const char *str,
           0,
           sizeof (struct TALER_Amount));
   /* skip leading whitespace */
-  while (isspace(str[0]))
+  while (isspace( (unsigned char) str[0]))
     str++;
   if ('\0' == str[0])
   {
diff --git a/src/wire/plugin_wire_sepa.c b/src/wire/plugin_wire_sepa.c
index 6300d8205..f0cef029f 100644
--- a/src/wire/plugin_wire_sepa.c
+++ b/src/wire/plugin_wire_sepa.c
@@ -309,7 +309,7 @@ validate_iban (const char *iban)
   nbuf = GNUNET_malloc ((len * 2) + 1);
   for (i=0, j=0; i < len; i++)
   {
-    if (isalpha ((int) ibancpy[i]))
+    if (isalpha ((unsigned char) ibancpy[i]))
     {
       if (2 != snprintf(&nbuf[j],
                         3,
@@ -326,7 +326,7 @@ validate_iban (const char *iban)
     j++;
   }
   for (j=0;'\0' != nbuf[j];j++)
-    GNUNET_assert (isdigit(nbuf[j]));
+    GNUNET_assert (isdigit( (unsigned char) nbuf[j]));
   GNUNET_assert (sizeof(dividend) >= 8);
   remainder = 0;
   for (i=0; i<j; i+=16)