diff --git a/cmd/ztest/Makefile.am b/cmd/ztest/Makefile.am
index 930a7ec3a8..cbfb95fd3b 100644
--- a/cmd/ztest/Makefile.am
+++ b/cmd/ztest/Makefile.am
@@ -3,7 +3,7 @@ include $(top_srcdir)/config/Rules.am
 # -Wnoformat-truncation to get rid of compiler warning for unchecked
 #  truncating snprintfs on gcc 7.1.1.
 AM_CFLAGS += $(DEBUG_STACKFLAGS) $(FRAME_LARGER_THAN) $(NO_FORMAT_TRUNCATION)
-AM_CPPFLAGS += -DDEBUG
+AM_CPPFLAGS += -DDEBUG -UNDEBUG
 
 DEFAULT_INCLUDES += \
 	-I$(top_srcdir)/include \