diff --git a/src/gui/gui-color.c b/src/gui/gui-color.c
index bc43ac44e..6aae528c3 100644
--- a/src/gui/gui-color.c
+++ b/src/gui/gui-color.c
@@ -683,9 +683,11 @@ gui_color_code_size (const char *string)
                     break;
                 case GUI_COLOR_EXTENDED_CHAR:
                     ptr_string++;
-                    if ((isdigit (ptr_string[0])) && (isdigit (ptr_string[1]))
-                        && (isdigit (ptr_string[2])) && (isdigit (ptr_string[3]))
-                        && (isdigit (ptr_string[4])))
+                    if ((isdigit ((unsigned char)ptr_string[0]))
+                        && (isdigit ((unsigned char)ptr_string[1]))
+                        && (isdigit ((unsigned char)ptr_string[2]))
+                        && (isdigit ((unsigned char)ptr_string[3]))
+                        && (isdigit ((unsigned char)ptr_string[4])))
                     {
                         ptr_string += 5;
                     }
@@ -713,8 +715,11 @@ gui_color_code_size (const char *string)
                     ptr_string++;
                     break;
                 default:
-                    if (isdigit (ptr_string[0]) && isdigit (ptr_string[1]))
+                    if (isdigit ((unsigned char)ptr_string[0])
+                        && isdigit ((unsigned char)ptr_string[1]))
+                    {
                         ptr_string += 2;
+                    }
                     break;
             }
             return ptr_string - string;
diff --git a/src/plugins/spell/spell.c b/src/plugins/spell/spell.c
index e9278e995..821fdcdd7 100644
--- a/src/plugins/spell/spell.c
+++ b/src/plugins/spell/spell.c
@@ -679,23 +679,23 @@ spell_skip_color_codes (char **string, char **result)
             /* IRC color code */
             weechat_string_dyn_concat (result, *string, 1);
             (*string)++;
-            if (isdigit (*string[0]))
+            if (isdigit ((unsigned char)*string[0]))
             {
                 /* foreground */
                 weechat_string_dyn_concat (result, *string, 1);
                 (*string)++;
-                if (isdigit (*string[0]))
+                if (isdigit ((unsigned char)*string[0]))
                 {
                     weechat_string_dyn_concat (result, *string, 1);
                     (*string)++;
                 }
             }
-            if ((*string[0] == ',') && (isdigit (*string[1])))
+            if ((*string[0] == ',') && (isdigit ((unsigned char)*string[1])))
             {
                 /* background */
                 weechat_string_dyn_concat (result, *string, 1);
                 (*string)++;
-                if (isdigit (*string[0]))
+                if (isdigit ((unsigned char)*string[0]))
                 {
                     weechat_string_dyn_concat (result, *string, 1);
                     (*string)++;