diff --git a/include/mbedtls/x509_csr.h b/include/mbedtls/x509_csr.h
index 8c31c09..c4bd7f1 100644
--- a/include/mbedtls/x509_csr.h
+++ b/include/mbedtls/x509_csr.h
@@ -349,7 +349,7 @@
  * \note            \p f_rng is used for the signature operation.
  */
 int mbedtls_x509write_csr_der(mbedtls_x509write_csr *ctx, unsigned char *buf, size_t size,
-                              int (*f_rng)(void *, unsigned char *, size_t),
+                              mbedtls_f_rng_t *f_rng,
                               void *p_rng);
 
 #if defined(MBEDTLS_PEM_WRITE_C)
@@ -368,7 +368,7 @@
  * \note            \p f_rng is used for the signature operation.
  */
 int mbedtls_x509write_csr_pem(mbedtls_x509write_csr *ctx, unsigned char *buf, size_t size,
-                              int (*f_rng)(void *, unsigned char *, size_t),
+                              mbedtls_f_rng_t *f_rng,
                               void *p_rng);
 #endif /* MBEDTLS_PEM_WRITE_C */
 #endif /* MBEDTLS_X509_CSR_WRITE_C */