diff --git a/tools/cooja/apps/mspsim/src/se/sics/cooja/mspmote/plugins/CodeUI.java b/tools/cooja/apps/mspsim/src/se/sics/cooja/mspmote/plugins/CodeUI.java
index ee8c91988..50faff24c 100644
--- a/tools/cooja/apps/mspsim/src/se/sics/cooja/mspmote/plugins/CodeUI.java
+++ b/tools/cooja/apps/mspsim/src/se/sics/cooja/mspmote/plugins/CodeUI.java
@@ -256,7 +256,8 @@ public class CodeUI extends JPanel {
     SwingUtilities.invokeLater(new Runnable() {
       public void run() {
         displayedFile = null;
-        codeEditor.setText(null);
+        codeEditor.setText("[no source displayed]");
+        codeEditor.setEnabled(false);
         codeEditorLines.clear();
         displayLine(-1, markCurrent);
       }
@@ -277,6 +278,7 @@ public class CodeUI extends JPanel {
         displayNoCode(markCurrent);
         return;
       }
+      codeEditor.setEnabled(true);
 
       String[] lines = data.split("\n");
       logger.info("Opening " + codeFile + " (" + lines.length + " lines)");