diff --git a/library/x509_crt.c b/library/x509_crt.c
index 00f3107..d72e2fb 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -679,8 +679,8 @@
     }
 
     if (*p != end) {
-        return MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-               MBEDTLS_ERR_ASN1_LENGTH_MISMATCH;
+        return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                                 MBEDTLS_ERR_ASN1_LENGTH_MISMATCH);
     }
 
     return 0;