diff --git a/programs/hash/md_hmac_demo.c b/programs/hash/md_hmac_demo.c
index 4b5aa3b..494e9ef 100644
--- a/programs/hash/md_hmac_demo.c
+++ b/programs/hash/md_hmac_demo.c
@@ -57,7 +57,7 @@
 const unsigned char key_bytes[32] = { 0 };
 
 /* Print the contents of a buffer in hex */
-void print_buf(const char *title, unsigned char *buf, size_t len)
+static void print_buf(const char *title, unsigned char *buf, size_t len)
 {
     printf("%s:", title);
     for (size_t i = 0; i < len; i++) {