diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 7a4d437914..c752d59d04 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -760,7 +760,9 @@ static int tls_prf_generic( mbedtls_md_type_t md_type,
 exit:
     mbedtls_md_free( &md_ctx );
 
-    mbedtls_platform_zeroize( tmp, tmp_len );
+    if ( tmp != NULL )
+        mbedtls_platform_zeroize( tmp, tmp_len );
+
     mbedtls_platform_zeroize( h_i, sizeof( h_i ) );
 
     mbedtls_free( tmp );