diff --git a/AvailableColumnsPanel.c b/AvailableColumnsPanel.c
index 8aedde24..0d9749e7 100644
--- a/AvailableColumnsPanel.c
+++ b/AvailableColumnsPanel.c
@@ -45,7 +45,7 @@ static HandlerResult AvailableColumnsPanel_eventHandler(Panel* super, int ch) {
       }
       default:
       {
-         if (ch < 255 && isalpha(ch))
+         if (0 < ch && ch < 255 && isalpha((unsigned char)ch))
             result = Panel_selectByTyping(super, ch);
          break;
       }
diff --git a/CategoriesPanel.c b/CategoriesPanel.c
index 8a1e16df..c12b3905 100644
--- a/CategoriesPanel.c
+++ b/CategoriesPanel.c
@@ -81,7 +81,7 @@ static HandlerResult CategoriesPanel_eventHandler(Panel* super, int ch) {
          break;
       }
       default:
-         if (ch < 255 && isalpha(ch))
+         if (0 < ch && ch < 255 && isalpha((unsigned char)ch))
             result = Panel_selectByTyping(super, ch);
          if (result == BREAK_LOOP)
             result = IGNORED;
diff --git a/ColumnsPanel.c b/ColumnsPanel.c
index 587175c8..318c7af8 100644
--- a/ColumnsPanel.c
+++ b/ColumnsPanel.c
@@ -91,7 +91,7 @@ static HandlerResult ColumnsPanel_eventHandler(Panel* super, int ch) {
       }
       default:
       {
-         if (ch < 255 && isalpha(ch))
+         if (0 < ch && ch < 255 && isalpha((unsigned char)ch))
             result = Panel_selectByTyping(super, ch);
          if (result == BREAK_LOOP)
             result = IGNORED;
diff --git a/IncSet.c b/IncSet.c
index e9e28934..b9a1d92d 100644
--- a/IncSet.c
+++ b/IncSet.c
@@ -137,7 +137,7 @@ bool IncSet_handleKey(IncSet* this, int ch, Panel* panel, IncMode_GetPanelValue
       if (size == 0) return true;
       IncMode_find(mode, panel, getPanelValue, 1);
       doSearch = false;
-   } else if (ch < 255 && isprint((char)ch)) {
+   } else if (0 < ch && ch < 255 && isprint((unsigned char)ch)) {
       if (mode->index < INCMODE_MAX) {
          mode->buffer[mode->index] = ch;
          mode->index++;
diff --git a/MainPanel.c b/MainPanel.c
index eb7e663d..4390dad6 100644
--- a/MainPanel.c
+++ b/MainPanel.c
@@ -79,7 +79,7 @@ static HandlerResult MainPanel_eventHandler(Panel* super, int ch) {
    } else if (ch != ERR && ch > 0 && ch < KEY_MAX && this->keys[ch]) {
       reaction |= (this->keys[ch])(this->state);
       result = HANDLED;
-   } else if (isdigit(ch)) {
+   } else if (0 < ch && ch < 255 && isdigit((unsigned char)ch)) {
       MainPanel_pidSearch(this, ch);
    } else {
       if (ch != ERR) {
diff --git a/Panel.c b/Panel.c
index b77483d9..315f3887 100644
--- a/Panel.c
+++ b/Panel.c
@@ -411,7 +411,7 @@ HandlerResult Panel_selectByTyping(Panel* this, int ch) {
       this->eventHandlerState = xCalloc(100, sizeof(char));
    char* buffer = this->eventHandlerState;
 
-   if (ch > 0 && ch < 255 && isalnum(ch)) {
+   if (0 < ch && ch < 255 && isalnum((unsigned char)ch)) {
       int len = strlen(buffer);
       if (len < 99) {
          buffer[len] = ch;