diff --git a/py/py.mk b/py/py.mk
index 0a98ccc00c..d864a7ed3d 100644
--- a/py/py.mk
+++ b/py/py.mk
@@ -266,6 +266,11 @@ $(HEADER_BUILD)/moduledefs.h: $(SRC_QSTR) $(QSTR_GLOBAL_DEPENDENCIES) | $(HEADER
 
 SRC_QSTR += $(HEADER_BUILD)/moduledefs.h
 
+# Standard C functions like memset need to be compiled with special flags so
+# the compiler does not optimise these functions in terms of themselves.
+CFLAGS_BUILTIN ?= -ffreestanding -fno-builtin -fno-lto
+$(BUILD)/lib/libc/string0.o: CFLAGS += $(CFLAGS_BUILTIN)
+
 # Force nlr code to always be compiled with space-saving optimisation so
 # that the function preludes are of a minimal and predictable form.
 $(PY_BUILD)/nlr%.o: CFLAGS += -Os