diff --git a/v4/Makefile b/v4/Makefile
index bd8a2905..cfee95fa 100644
--- a/v4/Makefile
+++ b/v4/Makefile
@@ -56,16 +56,20 @@ endif
 # AddressSanitizer and UndefinedBehaviorSanitizer at both widths and runs them.
 # The optimized run catches logic errors, this one catches the out-of-bounds
 # ring index and the width-dependent shift that a logic-only test cannot see.
+# -fno-sanitize-recover=all makes every UBSan report fatal: without it UBSan
+# prints the report and carries on, and the run still says "passed".
+#
+# Both binaries depend on this Makefile, so a flag change rebuilds them.
 define TEST_RULE
-$(BINDIR)/$(1)-$(notdir $(2)): $(2) $$(SRCS) $$(wildcard $(HERE)/include/v4/*.h)
+$(BINDIR)/$(1)-$(notdir $(2)): $(2) $$(SRCS) $$(wildcard $(HERE)/include/v4/*.h) $(HERE)/Makefile
 	@mkdir -p $(BINDIR)
 	$$(CC) $$(CFLAGS) -I$(HERE)/include -DV4_CELL_BITS=$(1) \
 		$(2) $$(SRCS) -o $$@
 
-$(BINDIR)/san-$(1)-$(notdir $(2)): $(2) $$(SRCS) $$(wildcard $(HERE)/include/v4/*.h)
+$(BINDIR)/san-$(1)-$(notdir $(2)): $(2) $$(SRCS) $$(wildcard $(HERE)/include/v4/*.h) $(HERE)/Makefile
 	@mkdir -p $(BINDIR)
 	$$(CC) $$(CSTD) $$(WARN) -O1 -g -fno-omit-frame-pointer \
-		-fsanitize=address,undefined -I$(HERE)/include -DV4_CELL_BITS=$(1) \
+		-fsanitize=address,undefined -fno-sanitize-recover=all -I$(HERE)/include -DV4_CELL_BITS=$(1) \
 		$(2) $$(SRCS) -o $$@
 endef