diff -r 29b0d0b61615 -r a6deb69743d4 src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp	Mon Nov 25 12:31:39 2019 +0100
+++ b/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp	Mon Nov 25 12:32:07 2019 +0100
@@ -629,7 +629,7 @@
 
 void ShenandoahHeap::decrease_used(size_t bytes) {
   assert(used() >= bytes, "never decrease heap size by more than we've left");
-  Atomic::sub(bytes, &_used);
+  Atomic::sub(&_used, bytes);
 }
 
 void ShenandoahHeap::increase_allocated(size_t bytes) {