diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 21e36b5..b5f7af5 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -304,7 +304,7 @@
     int ret;
     size_t len = 0;
     const mbedtls_asn1_sequence *last_ext = NULL;
-    mbedtls_asn1_sequence *ext;
+    const mbedtls_asn1_sequence *ext;
 
     memset( buf, 0, sizeof(buf) );