diff --git a/.gitignore b/.gitignore
index de375750..d8407bcb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,6 +6,7 @@ build
 git-hash
 *.tar.gz
 .*.swp
+.DS_Store
 
 # Gradle
 .gradle/
diff --git a/src/main/java/modbat/mbt/NoTransitionException.java b/src/main/java/modbat/mbt/NoTransitionException.java
new file mode 100644
index 00000000..5e73c49a
--- /dev/null
+++ b/src/main/java/modbat/mbt/NoTransitionException.java
@@ -0,0 +1,7 @@
+package modbat.mbt;
+
+class NoTransitionsException extends Exception {
+    NoTransitionsException(String message) {
+        super(message);
+    }
+}
diff --git a/src/main/scala/modbat/mbt/MBT.scala b/src/main/scala/modbat/mbt/MBT.scala
index b9740b5b..5baac448 100644
--- a/src/main/scala/modbat/mbt/MBT.scala
+++ b/src/main/scala/modbat/mbt/MBT.scala
@@ -213,7 +213,7 @@ object MBT {
     } catch {
       case e: ClassNotFoundException => {
 	Log.error("Class \"" + className + "\" not found.")
-	System.exit(1)
+	  throw e
       }
     }
   }
@@ -301,12 +301,13 @@ object MBT {
   // if modelInstance == null: initial model
   def launch(modelInstance: Model): MBT = {
     val model = mkModel(modelInstance)
+    
     if (Transition.pendingTransitions.isEmpty) {
       Log.error("Model " + model.getClass.getName + " has no transitions.")
       Log.error("Make sure at least one transition exists of type")
       Log.error("  \"a\" -> \"b\" := { code } // or, for an empty transition:")
       Log.error("  \"a\" -> \"b\" := skip")
-      System.exit(1)
+      throw new NoTransitionsException(model.getClass.getName)
     }
     val inst = new MBT(model, Transition.getTransitions)
     Transition.clear