4 files changed, 4 insertions, 4 deletions
diff --git a/filling.c b/filling.c
index 896fe41..b6d0ed1 100644
--- a/filling.c
+++ b/filling.c
@@ -1100,7 +1100,7 @@ static char *interpret_move(game_state *state, game_ui *ui, game_drawstate *ds,
         button = 0;
         break;
       default:
-        if (!isdigit(button)) return NULL;
+        if (button < '0' || button > '9') return NULL;
         button -= '0';
         if (button > (w == 2 && h == 2? 3: max(w, h))) return NULL;
     }
diff --git a/magnets.c b/magnets.c
index beeedf5..57bf37a 100644
--- a/magnets.c
+++ b/magnets.c
@@ -386,7 +386,7 @@ static char n2c(int num) { /* XXX cloned from singles.c */
 }
 
 static int c2n(char c) { /* XXX cloned from singles.c */
-    if (isdigit(c))
+    if (isdigit((unsigned char)c))
         return (int)(c - '0');
     else if (c >= 'a' && c <= 'z')
         return (int)(c - 'a' + 10);
diff --git a/signpost.c b/signpost.c
index b6db3a3..efed3f7 100644
--- a/signpost.c
+++ b/signpost.c
@@ -513,7 +513,7 @@ static void unpick_desc(game_params *params, char *desc,
         }
 
         c = *desc;
-        if (isdigit(c)) {
+        if (isdigit((unsigned char)c)) {
             num = (num*10) + (int)(c-'0');
             if (num > state->n) {
                 msg = "Number too large";
diff --git a/singles.c b/singles.c
index d8016ff..c20d0cf 100644
--- a/singles.c
+++ b/singles.c
@@ -324,7 +324,7 @@ static char n2c(int num) {
 }
 
 static int c2n(char c) {
-    if (isdigit(c))
+    if (isdigit((unsigned char)c))
         return (int)(c - '0');
     else if (c >= 'a' && c <= 'z')
         return (int)(c - 'a' + 10);