diff --git a/arangod/Aql/ClusterBlocks.cpp b/arangod/Aql/ClusterBlocks.cpp
index 45a4f9c929..311cc363b0 100644
--- a/arangod/Aql/ClusterBlocks.cpp
+++ b/arangod/Aql/ClusterBlocks.cpp
@@ -61,7 +61,7 @@ GatherBlock::GatherBlock(ExecutionEngine* engine, GatherNode const* en)
     : ExecutionBlock(engine, en),
       _sortRegisters(),
       _isSimple(en->getElements().empty()),
-      _heap(en->_sortmode == 'h' ? new Heap : nullptr ) {
+      _heap(en->_sortmode == 'h' ? new Heap : nullptr) {
 
   if (!_isSimple) {
     for (auto const& p : en->getElements()) {
@@ -155,13 +155,16 @@ int GatherBlock::initializeCursor(AqlItemBlock* items, size_t pos) {
     }
     _gatherBlockBuffer.clear();
     _gatherBlockPos.clear();
-
     _gatherBlockBuffer.reserve(_dependencies.size());
     _gatherBlockPos.reserve(_dependencies.size());
     for (size_t i = 0; i < _dependencies.size(); i++) {
       _gatherBlockBuffer.emplace_back();
       _gatherBlockPos.emplace_back(std::make_pair(i, 0));
     }
+    
+    if (_heap) {
+      _heap->clear();
+    }
   }
 
   if (_dependencies.empty()) {
@@ -276,21 +279,22 @@ AqlItemBlock* GatherBlock::getSome(size_t atLeast, size_t atMost) {
   TRI_ASSERT(_gatherBlockBuffer.size() == _dependencies.size());
   TRI_ASSERT(_gatherBlockBuffer.size() == _gatherBlockPos.size());
  
-  for (size_t i = 0; i < _dependencies.size(); i++) {
-    if (_gatherBlockBuffer.at(i).empty()) {
+  for (size_t i = 0; i < _dependencies.size(); ++i) {
+    if (_gatherBlockBuffer[i].empty()) {
       if (getBlock(i, atLeast, atMost)) {
         index = i;
-        _gatherBlockPos.at(i) = std::make_pair(i, 0);
+        _gatherBlockPos[i] = std::make_pair(i, 0);
       }
     } else {
       index = i;
     }
 
-    auto const& cur = _gatherBlockBuffer.at(i);
+    auto const& cur = _gatherBlockBuffer[i];
     if (!cur.empty()) {
-      available += cur.at(0)->size() - _gatherBlockPos.at(i).second;
-      for (size_t j = 1; j < cur.size(); j++) {
-        available += cur.at(j)->size();
+      TRI_ASSERT(cur[0]->size() >= _gatherBlockPos[i].second);
+      available += cur[0]->size() - _gatherBlockPos[i].second;
+      for (size_t j = 1; j < cur.size(); ++j) {
+        available += cur[j]->size();
       }
     }
   }
@@ -308,27 +312,27 @@ AqlItemBlock* GatherBlock::getSome(size_t atLeast, size_t atMost) {
 
   // comparison function
   OurLessThan ourLessThan(_trx, _gatherBlockBuffer, _sortRegisters);
-  auto ourGreater = [&ourLessThan](std::pair<std::size_t, std::size_t>& a
-                      ,std::pair<std::size_t, std::size_t>& b){
-    return ourLessThan(b,a);
+  auto ourGreater = [&ourLessThan](std::pair<std::size_t, std::size_t>& a, std::pair<std::size_t, std::size_t>& b) {
+    return ourLessThan(b, a);
   };
 
+  TRI_ASSERT(!_gatherBlockBuffer.at(index).empty());
   AqlItemBlock* example = _gatherBlockBuffer.at(index).front();
   size_t nrRegs = example->getNrRegs();
 
   // automatically deleted if things go wrong
   std::unique_ptr<AqlItemBlock> res(requestBlock(toSend, static_cast<arangodb::aql::RegisterId>(nrRegs)));
 
-  if (_heap && _heap->size() !=_dependencies.size() ){
+  if (_heap && _heap->size() != _dependencies.size()) {
     auto& heap = *_heap;
-    std::copy(_gatherBlockPos.begin(),_gatherBlockPos.end(),std::back_inserter(heap));
-    std::make_heap(heap.begin(), heap.end(),ourGreater);
+    std::copy(_gatherBlockPos.begin(), _gatherBlockPos.end(), std::back_inserter(heap));
+    std::make_heap(heap.begin(), heap.end(), ourGreater);
   }
 
   for (size_t i = 0; i < toSend; i++) {
     // get the next smallest row from the buffer . . .
     std::pair<size_t, size_t> val;
-    if(_heap){
+    if (_heap) {
       val = _heap->front();
     } else {
       val = *(std::min_element( _gatherBlockPos.begin(), _gatherBlockPos.end(), ourLessThan));
@@ -336,7 +340,8 @@ AqlItemBlock* GatherBlock::getSome(size_t atLeast, size_t atMost) {
 
     // copy the row in to the outgoing block . . .
     for (RegisterId col = 0; col < nrRegs; col++) {
-      AqlValue const& x( _gatherBlockBuffer.at(val.first).front()->getValueReference(val.second, col));
+      TRI_ASSERT(!_gatherBlockBuffer[val.first].empty());
+      AqlValue const& x(_gatherBlockBuffer[val.first].front()->getValueReference(val.second, col));
       if (!x.isEmpty()) {
         auto it = cache.find(x);
 
@@ -356,24 +361,25 @@ AqlItemBlock* GatherBlock::getSome(size_t atLeast, size_t atMost) {
     }
 
     _gatherBlockPos.at(val.first).second++;
-    if(_heap){
+    if (_heap) {
       auto& heap = *_heap;
-      std::pop_heap(heap.begin(), heap.end(),ourGreater); // remove element from heap but not from vector
+      std::pop_heap(heap.begin(), heap.end(), ourGreater); // remove element from heap but not from vector
       heap.back().second++; //advance position in itemblock of removed element before it is re-inserted later
     }
 
     // renew the _gatherBlockPos and clean up the buffer if necessary
-    if ( _gatherBlockPos.at(val.first).second == _gatherBlockBuffer.at(val.first).front()->size() ) {
-      AqlItemBlock* cur = _gatherBlockBuffer.at(val.first).front();
+    if (_gatherBlockPos.at(val.first).second == _gatherBlockBuffer.at(val.first).front()->size()) {
+      TRI_ASSERT(!_gatherBlockBuffer[val.first].empty());
+      AqlItemBlock* cur = _gatherBlockBuffer[val.first].front();
       returnBlock(cur);
-      _gatherBlockBuffer.at(val.first).pop_front();
-      _gatherBlockPos.at(val.first) = {val.first, 0}; // .second = 0 ?
+      _gatherBlockBuffer[val.first].pop_front();
+      _gatherBlockPos[val.first] = {val.first, 0}; 
 
-      if( _heap) {
+      if (_heap) {
         _heap->back().second = 0;
       }
 
-      if (_gatherBlockBuffer.at(val.first).empty()) {
+      if (_gatherBlockBuffer[val.first].empty()) {
         // if we pulled everything from the buffer, we need to fetch
         // more data for the shard for which we have no more local
         // values. 
@@ -384,8 +390,8 @@ AqlItemBlock* GatherBlock::getSome(size_t atLeast, size_t atMost) {
       }
     }
 
-    if(_heap) {
-      std::push_heap(_heap->begin(), _heap->end(),ourGreater); //re-insert element
+    if (_heap) {
+      std::push_heap(_heap->begin(), _heap->end(), ourGreater); //re-insert element
     }
   }
 
@@ -408,7 +414,7 @@ size_t GatherBlock::skipSome(size_t atLeast, size_t atMost) {
     auto skipped = _dependencies.at(_atDep)->skipSome(atLeast, atMost);
     while (skipped == 0 && _atDep < _dependencies.size() - 1) {
       _atDep++;
-      skipped = _dependencies.at(_atDep)->skipSome(atLeast, atMost);
+      skipped = _dependencies[_atDep]->skipSome(atLeast, atMost);
     }
     if (skipped == 0) {
       _done = true;
@@ -422,9 +428,9 @@ size_t GatherBlock::skipSome(size_t atLeast, size_t atMost) {
 
   // pull more blocks from dependencies . . .
   for (size_t i = 0; i < _dependencies.size(); i++) {
-    if (_gatherBlockBuffer.at(i).empty()) {
+    if (_gatherBlockBuffer[i].empty()) {
       if (getBlock(i, atLeast, atMost)) {
-        _gatherBlockPos.at(i) = std::make_pair(i, 0);
+        _gatherBlockPos[i] = std::make_pair(i, 0);
       }
     }
 
@@ -453,13 +459,14 @@ size_t GatherBlock::skipSome(size_t atLeast, size_t atMost) {
         _gatherBlockPos.begin(), _gatherBlockPos.end(), ourLessThan));
 
     // renew the _gatherBlockPos and clean up the buffer if necessary
-    _gatherBlockPos.at(val.first).second++;
-    if (_gatherBlockPos.at(val.first).second ==
-        _gatherBlockBuffer.at(val.first).front()->size()) {
-      AqlItemBlock* cur = _gatherBlockBuffer.at(val.first).front();
+    _gatherBlockPos[val.first].second++;
+    if (_gatherBlockPos[val.first].second ==
+        _gatherBlockBuffer[val.first].front()->size()) {
+      TRI_ASSERT(!_gatherBlockBuffer[val.first].empty());
+      AqlItemBlock* cur = _gatherBlockBuffer[val.first].front();
       returnBlock(cur);
-      _gatherBlockBuffer.at(val.first).pop_front();
-      _gatherBlockPos.at(val.first) = std::make_pair(val.first, 0);
+      _gatherBlockBuffer[val.first].pop_front();
+      _gatherBlockPos[val.first] = std::make_pair(val.first, 0);
     }
   }
 
@@ -477,7 +484,7 @@ bool GatherBlock::getBlock(size_t i, size_t atLeast, size_t atMost) {
   TRI_ASSERT(!_isSimple);
 
   std::unique_ptr<AqlItemBlock> docs(_dependencies.at(i)->getSome(atLeast, atMost));
-  if (docs != nullptr) {
+  if (docs != nullptr && docs->size() > 0) {
     _gatherBlockBuffer.at(i).emplace_back(docs.get());
     docs.release();
     return true;
@@ -499,6 +506,8 @@ bool GatherBlock::OurLessThan::operator()(std::pair<size_t, size_t> const& a,
   if (_gatherBlockBuffer[b.first].empty()) {
     return true;
   }
+  TRI_ASSERT(!_gatherBlockBuffer[a.first].empty());
+  TRI_ASSERT(!_gatherBlockBuffer[b.first].empty());
 
   for (auto const& reg : _sortRegisters) {
     // Fast path if there is no attributePath:
diff --git a/arangod/Aql/ClusterBlocks.h b/arangod/Aql/ClusterBlocks.h
index 51d62aca77..3f8fc312b9 100644
--- a/arangod/Aql/ClusterBlocks.h
+++ b/arangod/Aql/ClusterBlocks.h
@@ -34,7 +34,6 @@ namespace arangodb {
 namespace transaction {
 class Methods;
 }
-;
 struct ClusterCommResult;
 
 namespace aql {
@@ -117,7 +116,7 @@ class GatherBlock : public ExecutionBlock {
     std::vector<std::deque<AqlItemBlock*>>& _gatherBlockBuffer;
     std::vector<SortElementBlock>& _sortRegisters;
   };
-  using Heap = std::vector<std::pair<std::size_t,std::size_t>>;
+  using Heap = std::vector<std::pair<std::size_t, std::size_t>>;
   std::unique_ptr<Heap> _heap;
 };