Take care of empty lists to loop over.

arangod/Aql/ExecutionBlock.cpp

@@ -114,6 +114,12 @@ void ExecutionBlock::staticAnalysis (ExecutionBlock* super) {
     sq->getSubquery()->staticAnalysis(s);
   }
   v->reset();
+  // Now print the result:
+  for (auto it : _varOverview->varInfo) {
+    std::cout << "Variable ID:" << it.first << " RegisterId:" 
+              << it.second.registerId << " Depth:" << it.second.depth
+              << std::endl;
+  }
 }
 
 // Local Variables:

arangod/Aql/ExecutionBlock.h

@@ -1127,6 +1127,14 @@ namespace triagens {
             return nullptr;
           }
 
+          AqlItemBlock* res;
+
+          do {
+            // repeatedly try to get more stuff from upstream
+            // note that the value of the variable we have to loop over
+            // can contain zero entries, in which case we have to 
+            // try again!
+
             if (_buffer.empty()) {
               if (! ExecutionBlock::getBlock(DefaultBatchSize, DefaultBatchSize)) {
                 _done = true;
@@ -1173,10 +1181,14 @@ namespace triagens {
               }
             }
 
+            if (sizeInVar == 0) { 
+              res = nullptr;
+            }
+            else {
               size_t toSend = std::min(atMost, sizeInVar - _index); 
 
               //create the result
-          auto res = new AqlItemBlock(toSend, _varOverview->nrRegs[_depth]);
+              res = new AqlItemBlock(toSend, _varOverview->nrRegs[_depth]);
               
               inheritRegisters(cur, res, _pos);
 
@@ -1196,6 +1208,7 @@ namespace triagens {
                 // requirements
                 // Note that _index has been increased by 1 by getAqlValue!
               }
+            }
             if (_index == sizeInVar) {
               _index = 0;
               _thisblock = 0;
@@ -1207,6 +1220,7 @@ namespace triagens {
                 _pos = 0;
               }
             }
+          } while (res == nullptr);
           return res;
         }