diff --git a/tools/cooja/java/org/contikios/cooja/mote/memory/VarMemory.java b/tools/cooja/java/org/contikios/cooja/mote/memory/VarMemory.java
index c8866eeef..2559fe2f1 100644
--- a/tools/cooja/java/org/contikios/cooja/mote/memory/VarMemory.java
+++ b/tools/cooja/java/org/contikios/cooja/mote/memory/VarMemory.java
@@ -99,7 +99,11 @@ public class VarMemory extends Memory {
    * @return Variable address
    */
   public Symbol getVariable(String varName) throws UnknownVariableException {
-    return memIntf.getSymbolMap().get(varName);
+    Symbol sym = memIntf.getSymbolMap().get(varName);
+    if (sym == null) {
+      throw new UnknownVariableException(varName);
+    }
+    return sym;
   }
 
   /**