diff --git a/src/fsfw/container/HybridIterator.h b/src/fsfw/container/HybridIterator.h
index e8b24a3d..50a37988 100644
--- a/src/fsfw/container/HybridIterator.h
+++ b/src/fsfw/container/HybridIterator.h
@@ -10,16 +10,24 @@ class HybridIterator : public LinkedElement<T>::Iterator, public ArrayList<T, co
   HybridIterator() {}
 
   HybridIterator(typename LinkedElement<T>::Iterator *iter)
-      : LinkedElement<T>::Iterator(*iter), value(iter->value), linked(true) {}
+      : LinkedElement<T>::Iterator(*iter), value(iter->value), linked(true) {
+    if(iter != nullptr) {
+      value = iter->value;
+    }
+  }
 
   HybridIterator(LinkedElement<T> *start)
-      : LinkedElement<T>::Iterator(start), value(start->value), linked(true) {}
+      : LinkedElement<T>::Iterator(start), linked(true) {
+    if(start != nullptr) {
+      value = start->value;
+    }
+  }
 
   HybridIterator(typename ArrayList<T, count_t>::Iterator start,
                  typename ArrayList<T, count_t>::Iterator end)
       : ArrayList<T, count_t>::Iterator(start), value(start.value), linked(false), end(end.value) {
     if (value == this->end) {
-      value = NULL;
+      value = nullptr;
     }
   }
 
diff --git a/src/fsfw/globalfunctions/matching/MatchTree.h b/src/fsfw/globalfunctions/matching/MatchTree.h
index f7775d45..47e400da 100644
--- a/src/fsfw/globalfunctions/matching/MatchTree.h
+++ b/src/fsfw/globalfunctions/matching/MatchTree.h
@@ -179,6 +179,9 @@ class MatchTree : public SerializeableMatcherIF<T>, public BinaryTree<Serializea
   virtual ReturnValue_t cleanUpElement(iterator position) { return HasReturnvaluesIF::RETURN_OK; }
 
   bool matchSubtree(iterator iter, T number) {
+    if(iter == nullptr) {
+      return false;
+    }
     bool isMatch = iter->match(number);
     if (isMatch) {
       if (iter.left() == this->end()) {
diff --git a/src/fsfw/subsystem/Subsystem.cpp b/src/fsfw/subsystem/Subsystem.cpp
index a837bf83..767cfe39 100644
--- a/src/fsfw/subsystem/Subsystem.cpp
+++ b/src/fsfw/subsystem/Subsystem.cpp
@@ -30,11 +30,11 @@ ReturnValue_t Subsystem::checkSequence(HybridIterator<ModeListEntry> iter,
     return FALLBACK_SEQUENCE_DOES_NOT_EXIST;
   }
 
-  if (iter.value == NULL) {
+  if (iter.value ==nullptr) {
     return NO_TARGET_TABLE;
   }
 
-  for (; iter.value != NULL; ++iter) {
+  for (; iter.value != nullptr; ++iter) {
     if (!existsModeTable(iter->getTableId())) {
       return TABLE_DOES_NOT_EXIST;
     } else {
@@ -66,13 +66,18 @@ HybridIterator<ModeListEntry> Subsystem::getCurrentTable() {
 void Subsystem::performChildOperation() {
   if (isInTransition) {
     if (commandsOutstanding <= 0) {  // all children of the current table were commanded and replied
-      if (currentSequenceIterator.value == NULL) {  // we're through with this sequence
+      if (currentSequenceIterator.value == nullptr) {  // we're through with this sequence
         if (checkStateAgainstTable(currentTargetTable, targetSubmode) == RETURN_OK) {
           setMode(targetMode, targetSubmode);
           isInTransition = false;
           return;
         } else {
-          transitionFailed(TARGET_TABLE_NOT_REACHED, getSequence(targetMode)->getTableId());
+          Mode_t tableId = 0;
+          auto seq = getSequence(targetMode);
+          if(seq.value != nullptr) {
+             tableId = seq->getTableId();
+          }
+          transitionFailed(TARGET_TABLE_NOT_REACHED, tableId);
           return;
         }
       }
@@ -248,10 +253,13 @@ ReturnValue_t Subsystem::handleCommandMessage(CommandMessage *message) {
     case ModeSequenceMessage::READ_TABLE: {
       ReturnValue_t result;
       Mode_t table = ModeSequenceMessage::getSequenceId(message);
-      EntryPointer *entry = NULL;
+      EntryPointer *entry = nullptr;
       result = modeTables.find(table, &entry);
-      if (result != RETURN_OK) {
+      if (result != RETURN_OK or entry == nullptr) {
         replyToCommand(result, 0);
+        if(entry == nullptr) {
+          return result;
+        }
       }
 
       SerializeIF *elements[2];
diff --git a/src/fsfw/subsystem/Subsystem.h b/src/fsfw/subsystem/Subsystem.h
index 2c78c8cd..e0fafb51 100644
--- a/src/fsfw/subsystem/Subsystem.h
+++ b/src/fsfw/subsystem/Subsystem.h
@@ -1,7 +1,7 @@
 #ifndef FSFW_SUBSYSTEM_SUBSYSTEM_H_
 #define FSFW_SUBSYSTEM_SUBSYSTEM_H_
 
-#include <FSFWConfig.h>
+#include "fsfw/FSFW.h"
 
 #include "../container/FixedArrayList.h"
 #include "../container/FixedMap.h"