diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 7a4d437..c752d59 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -760,7 +760,9 @@
 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 );