diff --git a/build-aux/compiler.mak b/build-aux/compiler.mak
index 73dd5c67..6238feeb 100644
--- a/build-aux/compiler.mak
+++ b/build-aux/compiler.mak
@@ -124,9 +124,9 @@ endif
 DEFAULT_GENERIC_OPTLEVEL_BASE:=-Os -s
 DEFAULT_OPTLEVEL_FOR_BUILD:=$(DEFAULT_GENERIC_OPTLEVEL_BASE)
 ifeq ($(BUILD_IS_SORTIX),1)
-  DEFAULT_OPTLEVEL_FOR_BUILD+=
+  DEFAULT_OPTLEVEL_FOR_BUILD+=-fsanitize=undefined
 endif
 DEFAULT_OPTLEVEL:=$(DEFAULT_GENERIC_OPTLEVEL_BASE)
 ifeq ($(HOST_IS_SORTIX),1)
-  DEFAULT_OPTLEVEL+=
+  DEFAULT_OPTLEVEL+=-fsanitize=undefined
 endif