diff --git a/programs/x509/load_roots.c b/programs/x509/load_roots.c
index f0e6acf..d14537f 100644
--- a/programs/x509/load_roots.c
+++ b/programs/x509/load_roots.c
@@ -48,7 +48,7 @@
 } opt;
 
 
-int read_certificates(const char *const *filenames)
+static int read_certificates(const char *const *filenames)
 {
     mbedtls_x509_crt cas;
     int ret = 0;