diff --git a/vm/bump_allocator.hpp b/vm/bump_allocator.hpp
index 12c4453b63..5e8f3544df 100644
--- a/vm/bump_allocator.hpp
+++ b/vm/bump_allocator.hpp
@@ -36,6 +36,16 @@ template <typename Block> struct bump_allocator {
     else
       return 0;
   }
+
+  void flush() {
+    here = start;
+#ifdef FACTOR_DEBUG
+    /* In case of bugs, there may be bogus references pointing to the
+       memory space after the gc has run. Filling it with a pattern
+       makes accesses to such shadow data fail hard. */
+    memset_cell((void*)start, 0xbaadbaad, size);
+#endif
+  }
 };
 
 }
diff --git a/vm/data_heap.cpp b/vm/data_heap.cpp
index 5a5af548fa..89d1c4c8d0 100644
--- a/vm/data_heap.cpp
+++ b/vm/data_heap.cpp
@@ -68,10 +68,12 @@ template <typename Generation> void data_heap::clear_decks(Generation* gen) {
   memset(&decks[first_deck], 0, last_deck - first_deck);
 }
 
-void data_heap::reset_generation(nursery_space* gen) { gen->here = gen->start; }
+void data_heap::reset_generation(nursery_space* gen) {
+  gen->flush();
+}
 
 void data_heap::reset_generation(aging_space* gen) {
-  gen->here = gen->start;
+  gen->flush();
   clear_cards(gen);
   clear_decks(gen);
   gen->starts.clear_object_start_offsets();