diff --git a/AvailableColumnsPanel.c b/AvailableColumnsPanel.c
index d954da8c521e52dfd1986823e2a38db93fd9454e..6c4b5ef5ac58c11aca03fd51b2915f795808d122 100644
--- a/AvailableColumnsPanel.c
+++ b/AvailableColumnsPanel.c
@@ -55,7 +55,7 @@ static HandlerResult AvailableColumnsPanel_eventHandler(Panel* super, int ch) {
       }
       default:
       {
-         if (isalpha(ch))
+         if (ch < 255 && isalpha(ch))
             result = Panel_selectByTyping(super, ch);
          break;
       }
diff --git a/CategoriesPanel.c b/CategoriesPanel.c
index bf6ee543db276a36c27bf3d98910aff3317804c2..084f48e4cccb3ea0789ea3233e52ec710037bd52 100644
--- a/CategoriesPanel.c
+++ b/CategoriesPanel.c
@@ -102,7 +102,7 @@ static HandlerResult CategoriesPanel_eventHandler(Panel* super, int ch) {
          break;
       }
       default:
-         if (isalpha(ch))
+         if (ch < 255 && isalpha(ch))
             result = Panel_selectByTyping(super, ch);
          if (result == BREAK_LOOP)
             result = IGNORED;
diff --git a/ColumnsPanel.c b/ColumnsPanel.c
index f4eed99ee6ffeedbb53cdfac7ebe5bac3d01a96d..844e958002e87bcc2a152ec8d057cc91913d699b 100644
--- a/ColumnsPanel.c
+++ b/ColumnsPanel.c
@@ -70,7 +70,7 @@ static HandlerResult ColumnsPanel_eventHandler(Panel* super, int ch) {
       }
       default:
       {
-         if (isalpha(ch))
+         if (ch < 255 && isalpha(ch))
             result = Panel_selectByTyping(super, ch);
          if (result == BREAK_LOOP)
             result = IGNORED;
diff --git a/IncSet.c b/IncSet.c
index 40aa9233a89d266e9d0d5000b958192ea273e7b9..a77616026d8f26d4fda879333193b79b0552ba93 100644
--- a/IncSet.c
+++ b/IncSet.c
@@ -153,7 +153,7 @@ bool IncSet_handleKey(IncSet* this, int ch, Panel* panel, IncMode_GetPanelValue
          }
       }
       doSearch = false;
-   } else if (isprint((char)ch) && (mode->index < INCMODE_MAX)) {
+   } else if (ch < 255 && isprint((char)ch) && (mode->index < INCMODE_MAX)) {
       mode->buffer[mode->index] = ch;
       mode->index++;
       mode->buffer[mode->index] = 0;
diff --git a/Panel.c b/Panel.c
index 4355b1e2484814a5b38600798fc98a50dff5e504..681e9ca5fbb45fd3dd317067de32acebcf6cd695 100644
--- a/Panel.c
+++ b/Panel.c
@@ -427,7 +427,7 @@ HandlerResult Panel_selectByTyping(Panel* this, int ch) {
    if (!this->eventHandlerBuffer)
       this->eventHandlerBuffer = calloc(100, 1);
 
-   if (isalnum(ch)) {
+   if (ch < 255 && isalnum(ch)) {
       int len = strlen(this->eventHandlerBuffer);
       if (len < 99) {
          this->eventHandlerBuffer[len] = ch;
diff --git a/RichString.c b/RichString.c
index 51914b08c56202a1dba27218cb56641cb1485891..22ffecb51dcfb80772c5731e15b67482de70a13c 100644
--- a/RichString.c
+++ b/RichString.c
@@ -128,7 +128,7 @@ static inline void RichString_writeFrom(RichString* this, int attrs, const char*
    int newLen = from + len;
    RichString_setLen(this, newLen);
    for (int i = from, j = 0; i < newLen; i++, j++)
-      this->chptr[i] = (isprint(data_c[j]) ? data_c[j] : '?') | attrs;
+      this->chptr[i] = (data_c[j] >= 32 ? data_c[j] : '?') | attrs;
    this->chptr[newLen] = 0;
 }
 
diff --git a/htop.c b/htop.c
index 281d40ea8eacd51051f229952800a6f51fa692b7..62e49c9c1fe048f1bc0b9d1bb27576ea67a4d4af 100644
--- a/htop.c
+++ b/htop.c
@@ -852,7 +852,7 @@ int main(int argc, char** argv) {
          continue;
       }
       
-      if (isdigit((char)ch)) {
+      if (ch < 255 && isdigit((char)ch)) {
          if (Panel_size(panel) == 0) continue;
          pid_t pid = ch-48 + acc;
          for (int i = 0; i < ProcessList_size(pl); i++) {