diff --git a/Makefile b/Makefile
index 61e2b40..4aa2d25 100644
--- a/Makefile
+++ b/Makefile
@@ -8,11 +8,12 @@ PROGEXT = .elf
 SRCS = main.c
 SRCS+= strobe_rng_init.c
 
+CFLAGS+= -I$(.CURDIR)
 CFLAGS+= -DNDEBUG
 
 # Strobe
 .PATH: $(.CURDIR)/strobe
-CFLAGS = -I$(.CURDIR)/strobe
+CFLAGS+= -I$(.CURDIR)/strobe
 SRCS+= strobe.c \
 	x25519.c
 
@@ -28,6 +29,7 @@ CFLAGS+= -I$(STM32)/l151ccux
 CFLAGS+= -DSTM32L151xC
 
 OBJS = $(SRCS:C/.c$/.o/)
+CFLAGS+= -Werror -Wall
 
 .PHONY: all
 all: $(PROG)$(PROGEXT) $(PROG).list
diff --git a/strobe_rng_init.c b/strobe_rng_init.c
index ffeff92..4176fd7 100644
--- a/strobe_rng_init.c
+++ b/strobe_rng_init.c
@@ -2,7 +2,7 @@
 
 #include <strobe_rng_init.h>
 
-extern char rng_save;
+extern uint8_t rng_save;
 extern int rng_save_len;
 
 void
@@ -10,7 +10,16 @@ strobe_rng_init(void)
 {
 	int r;
 
+	/*
+	 * Seed RNG
+	 * On first boot, SRAM is uninitialized and randomness from
+	 * it is used.  On reset, the previously saved state is used.
+	 */
 	strobe_seed_prng(&rng_save, 2*1024);
 
+	/*
+	 * Save entropy for next reset.
+	 */
 	r = strobe_randomize(&rng_save, rng_save_len);
+	(void)r;
 }