--- old/src/hotspot/share/runtime/objectMonitor.inline.hpp	2019-04-25 12:04:43.264813917 -0400
+++ new/src/hotspot/share/runtime/objectMonitor.inline.hpp	2019-04-25 12:04:42.556813929 -0400
@@ -163,7 +163,9 @@
   // The decrement only needs to be MO_ACQ_REL since the reference
   // counter is volatile.
   Atomic::dec(&_ref_count);
-  guarantee(_ref_count >= 0, "sanity check: ref_count=%d", _ref_count);
+  // Can be negative as part of async deflation protocol.
+  guarantee(AsyncDeflateIdleMonitors || _ref_count >= 0,
+            "sanity check: ref_count=%d", _ref_count);
 }
 
 inline void ObjectMonitor::inc_ref_count() {
@@ -171,7 +173,9 @@
   // counter update is seen as soon as possible in a race with the
   // async deflation protocol.
   Atomic::inc(&_ref_count);
-  guarantee(_ref_count > 0, "sanity check: ref_count=%d", _ref_count);
+  // Can be negative as part of async deflation protocol.
+  guarantee(AsyncDeflateIdleMonitors || _ref_count > 0,
+            "sanity check: ref_count=%d", _ref_count);
 }
 
 inline jint ObjectMonitor::ref_count() const {