diff --git a/src/main/java/de/neemann/digital/draw/builder/CuplCreator.java b/src/main/java/de/neemann/digital/draw/builder/CuplCreator.java
index d80ffcf50..6cba5f76b 100644
--- a/src/main/java/de/neemann/digital/draw/builder/CuplCreator.java
+++ b/src/main/java/de/neemann/digital/draw/builder/CuplCreator.java
@@ -26,7 +26,7 @@ import static de.neemann.digital.analyse.expression.Operation.or;
  */
 public class CuplCreator implements BuilderInterface<CuplCreator> {
     private static final DateFormat DATE_FORMAT = new SimpleDateFormat("dd.MM.yyyy");
-    private static final ExpressionVisitor NOT_ALLOWED_VARIABLES_VISITOR = new NotAllowedVariablesVisitor();
+    private static final NotAllowedVariablesVisitor NOT_ALLOWED_VARIABLES_VISITOR = new NotAllowedVariablesVisitor();
 
     private final StringBuilder expressions;
     private final String projectName;
@@ -63,7 +63,7 @@ public class CuplCreator implements BuilderInterface<CuplCreator> {
 
     @Override
     public CuplCreator addExpression(String name, Expression expression) throws BuilderException {
-        outVars.add(name);
+        addOutVar(name);
         addToStr(name, expression);
         return this;
     }
@@ -71,11 +71,16 @@ public class CuplCreator implements BuilderInterface<CuplCreator> {
     @Override
     public CuplCreator addState(String name, Expression expression) throws BuilderException {
         sequential = true;
-        outVars.add(name);
+        addOutVar(name);
         addToStr(name + ".D", expression);
         return this;
     }
 
+    private void addOutVar(String name) {
+        NOT_ALLOWED_VARIABLES_VISITOR.check(name);
+        outVars.add(name);
+    }
+
     private void addToStr(String name, Expression expression) throws BuilderException {
         expression.traverse(vars);
         expression.traverse(NOT_ALLOWED_VARIABLES_VISITOR);
@@ -151,12 +156,16 @@ public class CuplCreator implements BuilderInterface<CuplCreator> {
         public boolean visit(Expression expression) {
             if (expression instanceof Variable) {
                 Variable v = (Variable) expression;
-                if (notAllowed.contains(v.getIdentifier()))
-                    throw new RuntimeException(Lang.get("err_varNotAllowedInCUPL_N", v.getIdentifier()));
+                check(v.getIdentifier());
             }
             return true;
         }
 
+        private void check(String v) {
+            if (notAllowed.contains(v))
+                throw new RuntimeException(Lang.get("err_varNotAllowedInCUPL_N", v));
+        }
+
     }
 
     /**
diff --git a/src/test/java/de/neemann/digital/draw/builder/CuplCreatorTest.java b/src/test/java/de/neemann/digital/draw/builder/CuplCreatorTest.java
index 4ee4bd065..8ff7007fb 100644
--- a/src/test/java/de/neemann/digital/draw/builder/CuplCreatorTest.java
+++ b/src/test/java/de/neemann/digital/draw/builder/CuplCreatorTest.java
@@ -57,17 +57,30 @@ public class CuplCreatorTest extends TestCase {
 
     public void testCUPLBuilderInvalidVars() throws Exception {
         Variable y0 = new Variable("D");  // D is not allowed in CUPL
-        Variable y1 = new Variable("Y_1");
 
         // counter
         Expression y0s = not(y0);
-        Expression y1s = or(and(not(y0), y1), and(y0, not(y1)));
 
         try {
             new CuplCreator("test", "user", new Date(0))
-                    .addState("Y_0", y0s)
-                    .addState("Y_1", y1s)
-                    .addExpression("A", and(y0, y1));
+                    .addState("Y_0", y0s);
+
+            assertTrue(false);
+        } catch (RuntimeException e) {
+            assertTrue(true);
+        }
+    }
+
+    public void testCUPLBuilderInvalidVars2() throws Exception {
+        Variable y0 = new Variable("Y_0");  // D is not allowed in CUPL
+
+        // counter
+        Expression y0s = not(y0);
+
+        try {
+            new CuplCreator("test", "user", new Date(0))
+                    .addExpression("D", y0s)
+                    .writeTo(System.out);
 
             assertTrue(false);
         } catch (RuntimeException e) {