diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index ac4d96d..9c4be53 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -276,6 +276,10 @@
 #endif /* MBEDTLS_SSL_PROTO_TLS1_2 &&
           MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED */
 
+typedef int  mbedtls_ssl_tls_prf_cb( const unsigned char *secret, size_t slen,
+                                     const char *label,
+                                     const unsigned char *random, size_t rlen,
+                                     unsigned char *dstbuf, size_t dlen );
 /*
  * This structure contains the parameters only needed during handshake.
  */
@@ -425,9 +429,7 @@
     void (*update_checksum)(mbedtls_ssl_context *, const unsigned char *, size_t);
     void (*calc_verify)(mbedtls_ssl_context *, unsigned char *);
     void (*calc_finished)(mbedtls_ssl_context *, unsigned char *, int);
-    int  (*tls_prf)(const unsigned char *, size_t, const char *,
-                    const unsigned char *, size_t,
-                    unsigned char *, size_t);
+    mbedtls_ssl_tls_prf_cb *tls_prf;
 
     mbedtls_ssl_ciphersuite_t const *ciphersuite_info;