diff --git a/lib/Makefile b/lib/Makefile
index 2bc31a28..aaad7293 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -24,7 +24,7 @@ PREFIX		?= arm-elf
 CC		= $(PREFIX)-gcc
 AR		= $(PREFIX)-ar
 CFLAGS		= -Os -g -Wall -Wextra -I../include -fno-common \
-		  -mcpu=cortex-m3 -mthumb
+		  -mcpu=cortex-m3 -mthumb -Wstrict-prototypes
 # ARFLAGS	= rcsv
 ARFLAGS		= rcs
 OBJS		= gpio.o rcc.o