--- a/make/hotspot/lib/JvmFlags.gmk	Thu Feb 07 10:26:32 2019 +0100
+++ b/make/hotspot/lib/JvmFlags.gmk	Thu Feb 07 12:35:45 2019 +0100
@@ -74,7 +74,7 @@
   endif
 else ifeq ($(DEBUG_LEVEL), fastdebug)
   JVM_CFLAGS_DEBUGLEVEL := -DASSERT
-  ifeq ($(filter $(OPENJDK_TARGET_OS), windows aix), )
+  ifeq ($call isTargetOs, windows aix), false)
     # NOTE: Old build did not define CHECK_UNHANDLED_OOPS on Windows and AIX.
     JVM_CFLAGS_DEBUGLEVEL += -DCHECK_UNHANDLED_OOPS
   endif