--- old/src/hotspot/share/gc/g1/g1CollectionSet.cpp	2019-11-25 14:31:31.176545873 +0100
+++ new/src/hotspot/share/gc/g1/g1CollectionSet.cpp	2019-11-25 14:31:30.790533706 +0100
@@ -147,6 +147,11 @@
 void G1CollectionSet::start_incremental_building() {
   assert(_collection_set_cur_length == 0, "Collection set must be empty before starting a new collection set.");
   assert(_inc_build_state == Inactive, "Precondition");
+#ifdef ASSERT
+  for (size_t i = 0; i < _collection_set_max_length; i++) {
+    _inc_collection_set_stats[i].reset();
+  }
+#endif
 
   _inc_bytes_used_before = 0;
 
@@ -251,6 +256,7 @@
   _inc_recorded_rs_length_diff += rs_length_diff;
 
   double old_non_copy_time = stat->_non_copy_time_ms;
+  assert(old_non_copy_time >= 0.0, "Non copy time for region %u not initialized yet, is %.3f", hr->hrm_index(), old_non_copy_time);
   double new_non_copy_time = predict_region_non_copy_time_ms(hr);
   double non_copy_time_ms_diff = new_non_copy_time - old_non_copy_time;