diff --git a/library/ssl_tls12_client.c b/library/ssl_tls12_client.c
index 895896c..91cad8b 100644
--- a/library/ssl_tls12_client.c
+++ b/library/ssl_tls12_client.c
@@ -3096,7 +3096,8 @@
          * - the PSK itself
          */
         unsigned char *p = ssl->handshake->premaster;
-        unsigned char *p_end = p + sizeof( ssl->handshake->premaster );
+        const unsigned char* const p_end = p +
+                                sizeof( ssl->handshake->premaster );
         size_t zlen = 0;
 
         /* Perform ECDH computation after the uint16 reserved for the length */