diff --git a/commandedit.cpp b/commandedit.cpp
--- a/commandedit.cpp
+++ b/commandedit.cpp
@@ -60,7 +60,9 @@ CommandEdit::CommandEdit(QWidget *parent
     QLineEdit(parent)
 {
     hexValidator = new HexCommandValidator(this);
+    asciiValidator = new QRegExpValidator(QRegExp("[\\x0000-\\x007F]+"));
     ascii_mode = true;
+    setValidator(asciiValidator);
 }
 
 CommandEdit::~CommandEdit()
@@ -73,7 +75,7 @@ void CommandEdit::setMode(bool ascii)
     ascii_mode = ascii;
     if (ascii)
     {
-        setValidator(0);
+        setValidator(asciiValidator);
     }
     else
     {
diff --git a/commandedit.h b/commandedit.h
--- a/commandedit.h
+++ b/commandedit.h
@@ -35,6 +35,7 @@ public:
 private:
     bool ascii_mode;
     QValidator* hexValidator;
+    QValidator* asciiValidator;
 
 protected:
     void keyPressEvent(QKeyEvent * event) Q_DECL_OVERRIDE;