diff --git a/src/toolbars/SpectralSelectionBar.cpp b/src/toolbars/SpectralSelectionBar.cpp
index 713e62a08..fb443452c 100644
--- a/src/toolbars/SpectralSelectionBar.cpp
+++ b/src/toolbars/SpectralSelectionBar.cpp
@@ -158,12 +158,14 @@ void SpectralSelectionBar::Populate()
 
    mLowCtrl = new NumericTextCtrl(
       NumericConverter::FREQUENCY, this, OnLowID, frequencyFormatName, 0.0);
+   mLowCtrl->SetInvalidValue(SelectedRegion::UndefinedFrequency);
    mLowCtrl->SetName(_("Low Frequency:"));
    mLowCtrl->EnableMenu();
    subSizer->Add(mLowCtrl, 0, wxALIGN_CENTER_VERTICAL | wxRIGHT, 5);
 
    mHighCtrl = new NumericTextCtrl(
       NumericConverter::FREQUENCY, this, OnHighID, frequencyFormatName, 0.0);
+   mHighCtrl->SetInvalidValue(SelectedRegion::UndefinedFrequency);
    mHighCtrl->SetName(wxString(_("High Frequency:")));
    mHighCtrl->EnableMenu();
    subSizer->Add(mHighCtrl, 0, wxALIGN_CENTER_VERTICAL | wxRIGHT, 0);
diff --git a/src/widgets/NumericTextCtrl.cpp b/src/widgets/NumericTextCtrl.cpp
index a17d65d95..cde2fe674 100644
--- a/src/widgets/NumericTextCtrl.cpp
+++ b/src/widgets/NumericTextCtrl.cpp
@@ -533,6 +533,7 @@ NumericConverter::NumericConverter(Type type,
 {
    ResetMinValue();
    ResetMaxValue();
+   mInvalidValue = -1.0;
 
    mDefaultNdx = 0;
 
@@ -866,7 +867,7 @@ void NumericConverter::ControlsToValue()
 
    if (mFields.GetCount() > 0 &&
       mValueString.Mid(mFields[0].pos, 1) == wxChar('-')) {
-      mValue = -1;
+      mValue = mInvalidValue;
       return;
    }
 
@@ -1169,6 +1170,7 @@ NumericTextCtrl::NumericTextCtrl(NumericConverter::Type type,
    mAutoPos(autoPos)
    , mType(type)
 {
+   mAllowInvalidValue = false;
 
    mDigitBoxW = 10;
    mDigitBoxH = 16;
@@ -1211,6 +1213,7 @@ NumericTextCtrl::~NumericTextCtrl()
 
 // Set the focus to the first (left-most) non-zero digit
 // If all digits are zero, the right-most position is focused
+// If all digits are hyphens (invalid), the left-most position is focused
 void NumericTextCtrl::UpdateAutoFocus()
 {
    if (!mAutoPos)
@@ -1280,6 +1283,15 @@ void NumericTextCtrl::EnableMenu(bool enable)
    Fit();
 }
 
+void NumericTextCtrl::SetInvalidValue(double invalidValue)
+{
+   const bool wasInvalid = mAllowInvalidValue && (mValue == mInvalidValue);
+   mAllowInvalidValue = true;
+   mInvalidValue = invalidValue;
+   if (wasInvalid)
+      SetValue(invalidValue);
+}
+
 bool NumericTextCtrl::Layout()
 {
    unsigned int i, j;
@@ -1596,6 +1608,7 @@ void NumericTextCtrl::OnCaptureKey(wxCommandEvent &event)
       case WXK_TAB:
       case WXK_RETURN:
       case WXK_NUMPAD_ENTER:
+      case '-':
          return;
 
       default:
@@ -1618,6 +1631,7 @@ void NumericTextCtrl::OnKeyUp(wxKeyEvent &event)
       keyCode -= WXK_NUMPAD0 - '0';
 
    if ((keyCode >= '0' && keyCode <= '9') ||
+       (keyCode == '-') ||
        (keyCode == WXK_BACK) ||
        (keyCode == WXK_UP) ||
        (keyCode == WXK_DOWN)) {
@@ -1661,6 +1675,11 @@ void NumericTextCtrl::OnKeyDown(wxKeyEvent &event)
       Updated();
    }
 
+   else if (!mReadOnly && keyCode == '-') {
+      if (mAllowInvalidValue)
+         SetValue(mInvalidValue);
+   }
+
    else if (!mReadOnly && keyCode == WXK_BACK) {
       // Moves left, replaces that char with '0', stays there...
       mFocusedDigit--;
diff --git a/src/widgets/NumericTextCtrl.h b/src/widgets/NumericTextCtrl.h
index 87cfec599..9e1e20aaa 100644
--- a/src/widgets/NumericTextCtrl.h
+++ b/src/widgets/NumericTextCtrl.h
@@ -75,6 +75,7 @@ public:
    void ResetMinValue();
    void SetMaxValue(double maxValue);
    void ResetMaxValue();
+
    double GetValue();
 
    wxString GetString();
@@ -101,6 +102,7 @@ protected:
 
    double         mMinValue;
    double         mMaxValue;
+   double         mInvalidValue;
 
    wxString       mFormatString;
 
@@ -154,6 +156,12 @@ class NumericTextCtrl: public wxControl, public NumericConverter
    void SetReadOnly(bool readOnly = true);
    void EnableMenu(bool enable = true);
 
+   // The text control permits typing '-' to make the value invalid only if this
+   // function has previously been called.
+   // Maybe you want something other than the default of -1 to indicate the invalid value
+   // this control returns to the program, so you can specify.
+   void SetInvalidValue(double invalidValue);
+
    int GetFocusedField() { return mLastField; }
    int GetFocusedDigit() { return mFocusedDigit; }
 
@@ -207,6 +215,8 @@ private:
 
    NumericConverter::Type mType;
 
+   bool           mAllowInvalidValue;
+
    DECLARE_EVENT_TABLE()
 };