diff --git a/library/x509_crt.c b/library/x509_crt.c
index 88d7f04..525d250 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -359,6 +359,9 @@
         /* Allocate and assign next pointer */
         if( cur->buf.p != NULL )
         {
+            if( cur->next != NULL )
+                return( POLARSSL_ERR_X509_INVALID_EXTENSIONS );
+
             cur->next = (asn1_sequence *) polarssl_malloc(
                  sizeof( asn1_sequence ) );