diff --git a/library/x509_crt.c b/library/x509_crt.c
index 3b75bb5..18db3b2 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -2989,7 +2989,7 @@
     for( parent_crt = candidates; parent_crt != NULL;
          parent_crt = parent_crt->next )
     {
-        int parent_valid, parent_match, path_len_ok;
+        volatile int parent_valid, parent_match, path_len_ok;
 
 #if defined(MBEDTLS_ECDSA_C) && defined(MBEDTLS_ECP_RESTARTABLE)
 check_signature: