diff --git a/CHANGELOG b/CHANGELOG
index c75c79418ecb..bb069c337bbf 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,6 +1,9 @@
 devel
 -----
 
+* Fix numeric overflow in AQL WINDOW node cost estimation if the number of
+  preceding rows was set to `unbounded`.
+
 * Updated ArangoDB Starter to 0.15.1-preview-3.
 
 * Added garbage collection for finished and failed Pregel conductors.
diff --git a/arangod/Aql/WindowNode.cpp b/arangod/Aql/WindowNode.cpp
index 88c0972ae0d4..dd0f8a089615 100644
--- a/arangod/Aql/WindowNode.cpp
+++ b/arangod/Aql/WindowNode.cpp
@@ -492,8 +492,14 @@ CostEstimate WindowNode::estimateCost() const {
   // we never return more rows than above
   CostEstimate estimate = _dependencies.at(0)->getCost();
   if (_rangeVariable == nullptr) {
-    int64_t numRows = 1 + _bounds.numPrecedingRows() + _bounds.numFollowingRows();
-    estimate.estimatedCost += double(numRows * numRows) * _aggregateVariables.size();
+    uint64_t numRows = 1;
+    if ( _bounds.unboundedPreceding()) {
+      numRows += estimate.estimatedNrItems;
+    } else {
+      numRows += std::min<uint64_t>(estimate.estimatedNrItems, _bounds.numPrecedingRows());
+    }
+    numRows += _bounds.numFollowingRows();
+    estimate.estimatedCost += double(numRows) * double(numRows) * _aggregateVariables.size();
   } else {  // guestimate
     estimate.estimatedCost += 4 * _aggregateVariables.size();
   }