diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 8dbae7aaa..6dabd3409 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1849,7 +1849,7 @@ int mbedtls_ssl_psk_derive_premaster( mbedtls_ssl_context *ssl, mbedtls_key_exch
     unsigned char *p = ssl->handshake->premaster;
     unsigned char *end = p + sizeof( ssl->handshake->premaster );
     const unsigned char *psk;
-    size_t psk_len;
+    size_t psk_len = 0;
 
     if( mbedtls_ssl_get_psk( ssl, &psk, &psk_len )
             == MBEDTLS_ERR_SSL_PRIVATE_KEY_REQUIRED )