diff --git a/qucs/diagrams/diagram.cpp b/qucs/diagrams/diagram.cpp
index ff53c9ab6..54354991f 100644
--- a/qucs/diagrams/diagram.cpp
+++ b/qucs/diagrams/diagram.cpp
@@ -859,7 +859,10 @@ int Graph::loadDatFile(const QString &fileName) {
     qDeleteAll(g->mutable_axes());
     g->mutable_axes().clear();
     g->countY = 0;
-    delete[] g->cPointsY;
+    if (g->cPointsY != nullptr) {
+        delete[] g->cPointsY;
+        g->cPointsY = nullptr;
+    }
     if (Variable.isEmpty()) return 0;
 
 #if 0 // FIXME encapsulation. implement digital waves later.
diff --git a/qucs/diagrams/graph.cpp b/qucs/diagrams/graph.cpp
index 18785bf2b..72b7a7708 100644
--- a/qucs/diagrams/graph.cpp
+++ b/qucs/diagrams/graph.cpp
@@ -48,8 +48,11 @@ Graph::Graph(Diagram const* d, const QString& _Line) :
 
 Graph::~Graph()
 {
+  if (cPointsY != nullptr) {
     delete[] cPointsY;
-    qDeleteAll(cPointsX);
+    cPointsY = nullptr;
+  }
+  qDeleteAll(cPointsX);
 }
 
 // ---------------------------------------------------------------------