diff --git a/qucs/qucs/dialogs/sweepdialog.cpp b/qucs/qucs/dialogs/sweepdialog.cpp
index 4dd940e2d4..4a2268f316 100644
--- a/qucs/qucs/dialogs/sweepdialog.cpp
+++ b/qucs/qucs/dialogs/sweepdialog.cpp
@@ -46,12 +46,16 @@ mySpinBox::mySpinBox(int Min, int Max, int Step, double *Val, QWidget *Parent)
 using namespace std;
 QString mySpinBox::textFromValue(int Val) const
 {
+  if (Values == NULL) return "";
+
   cout<<"Values + Val"<<*(Values+Val)<<endl;
   return QString::number(*(Values+Val));
 }
 
 QValidator::State mySpinBox::validate ( QString & text, int & pos ) const
 {
+  if (Values == NULL) return QValidator::Invalid;
+
   if(pos>ValueSize)return QValidator::Invalid; 
   if(QString::number(*(Values+pos))==text)
   return QValidator::Acceptable;
@@ -128,15 +132,18 @@ void SweepDialog::slotNewValue(int)
   DataX *pD = pGraph->cPointsX.first();
   int Factor = 1, Index = 0;
   QList<mySpinBox *>::const_iterator it;
-  for(it = BoxList.constBegin(); it != BoxList.constEnd(); it++) {
+
+  for (it = BoxList.constBegin(); it != BoxList.constEnd(); it++) {
     Index  += (*it)->value() * Factor;
     Factor *= pD->count;
   }
+
   Index *= 2;  // because of complex values
 
   QList<Node *>::iterator node_it;
-  QList<double *>::const_iterator value_it;
-  for(node_it = NodeList.begin(); node_it != NodeList.end(); node_it++) {
+  QList<double *>::const_iterator value_it = ValueList.begin();
+
+  for (node_it = NodeList.begin(); node_it != NodeList.end(); node_it++) {
     (*node_it)->Name = misc::num2str(*((*value_it)+Index));
     (*node_it)->Name += ((*node_it)->x1 & 0x10)? "A" : "V";
     value_it++;
diff --git a/qucs/qucs/dialogs/sweepdialog.h b/qucs/qucs/dialogs/sweepdialog.h
index d77e74e416..2b2af39888 100644
--- a/qucs/qucs/dialogs/sweepdialog.h
+++ b/qucs/qucs/dialogs/sweepdialog.h
@@ -42,7 +42,7 @@ class mySpinBox : public QSpinBox {
   QValidator::State validate ( QString & text, int & pos ) const;
 
 private:
-  double *Values;
+  double *Values = NULL;
   int ValueSize;
 };