diff --git a/src/main/scala/scim/components/Simulation.scala b/src/main/scala/scim/components/Simulation.scala
index f9dab7b..0e39c3e 100644
--- a/src/main/scala/scim/components/Simulation.scala
+++ b/src/main/scala/scim/components/Simulation.scala
@@ -49,7 +49,7 @@ class Simulation(config: Configuration) {
     sharedMemory.publish(time)
   }
 
-  private def simGroups(time: Time, cell: Cell, groups: Map[Group, List[(Group, Sensor, Option[GroundTruth])]], configTruths: Option[Map[Group, Truth]], PrevTruths: Map[Group, Truth], resultTruths: Map[Group, Truth], resultSensors: Map[Group, Vector[SensorResult]]): Unit = {
+  private def simGroups(time: Time, cell: Cell, groups: Map[Group, List[(Group, Sensor, Option[GroundTruth])]], configTruths: Option[Map[Group, Truth]], prevTruths: Map[Group, Truth], resultTruths: Map[Group, Truth], resultSensors: Map[Group, Vector[SensorResult]]): Unit = {
     // TODO: possible optimisation (see text below)
     // this can be further optimised as a child is only dependent on its parents ground truth and could run
     // at the same time as the parent in a different thread. This would allow the child to start running as soon
@@ -63,15 +63,36 @@ class Simulation(config: Configuration) {
       val group = input._1
       val truthGroup = group.getParentOrGroup // make sure the correct ground truth gets passed
       val groupList = input._2
-      // overwrite truth with value from configuration if set for a non child group at this time (children just get results of their parent)
-      val prevTruth = if(group.parent.isEmpty && !configTruths.isEmpty && configTruths.get.contains(group)) configTruths.get(group) else PrevTruths(truthGroup)
       groupList.foreach(element => {
         val subGroup = element._1
-        debugf(DebugType.DEBUG, "simGroups: Time: %s | Cell: %s | Group: %s | Groupelement: %s", time.toString, cell.toString, group.toString, element._1.name)
+        val subGroupParent = sharedMemory.groups(subGroup.parent.getOrElse(group.name))
+        // overwrite truth with value from configuration if this is the root group
+        val prevTruth = if(subGroup.parent.isEmpty && !configTruths.isEmpty && configTruths.get.contains(subGroup)) {
+          configTruths.get(subGroup)
+          // make sure parent actually calculates a ground truth
+        } else if(prevTruths.contains(subGroupParent) && subGroupParent.sensorParent.isEmpty) {
+          prevTruths(subGroupParent)
+        } else {
+          // this can happen if sensor parent has another sensor parent
+          // TODO: A sensor with parent should not have children (warn in parser)
+          debugf(DebugType.WARNING, "Group [%s] parent does not have a ground truth, using default Root Group [%s]", subGroup.toString, group.toString())
+          prevTruths(group)
+        }
         // pass either subGroup truth, if truth is available for subGroup, or the sensor parents truth
-        val resTruth = if(subGroup.sensorParent.isEmpty) resultTruths(subGroup) else resultTruths(sharedMemory.groups(subGroup.parent.getOrElse(subGroup.name)))
+        val resTruth = if(subGroup.sensorParent.isEmpty) {
+          resultTruths(subGroup)
+        } else if(resultTruths.contains(subGroupParent) && subGroupParent.sensorParent.isEmpty) {
+          resultTruths(subGroupParent)
+        } else {
+        // this can happen if sensor parent has another sensor parent
+        // TODO: A sensor with parent should not have children (warn in parser)
+          debugf(DebugType.WARNING, "Group [%s] parent does not have a ground truth, using Root Group [%s]", subGroup.toString, group.toString())
+          resultTruths(group)
+        }
+
         // construct dummy result for sensors with count 0
         val restSens = if(resultSensors.contains(subGroup)) resultSensors(subGroup) else Vector(SensorResult())
+        debugf(DebugType.DEBUG, "simGroups: Time: %s | Cell: %s | Root: %s | Groupelement: %s | prevTruth: %s | resTruth: %s", time.toString, cell.toString, group.toString, element._1.name, prevTruth.peak(), resTruth.peak())
         simulate(time, cell, prevTruth, resTruth, restSens, element)
       })
     }