diff --git a/library/bignum.c b/library/bignum.c
index 1c3bfe0e5b..61aa961c6e 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2132,7 +2132,7 @@ cleanup:
 /*
  * Pseudo-primality test: small factors, then Miller-Rabin
  */
-int mpi_is_prime_internal( const mbedtls_mpi *X, int rounds,
+static int mpi_is_prime_internal( const mbedtls_mpi *X, int rounds,
                   int (*f_rng)(void *, unsigned char *, size_t),
                   void *p_rng )
 {