diff --git a/programs/psa/hmac_demo.c b/programs/psa/hmac_demo.c
index f44f9a7..683f3e5 100644
--- a/programs/psa/hmac_demo.c
+++ b/programs/psa/hmac_demo.c
@@ -59,7 +59,7 @@
 const unsigned char key_bytes[32] = { 0 };
 
 /* Print the contents of a buffer in hex */
-void print_buf(const char *title, uint8_t *buf, size_t len)
+static void print_buf(const char *title, uint8_t *buf, size_t len)
 {
     printf("%s:", title);
     for (size_t i = 0; i < len; i++) {