diff --git a/library/x509_crt.c b/library/x509_crt.c
index 059b60f..11eb7cf 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -852,7 +852,7 @@
      * one or more PEM certificates.
      */
 #if defined(MBEDTLS_PEM_PARSE_C)
-    if( buf[buflen - 1] == '\0' &&
+    if( buflen != 0 && buf[buflen - 1] == '\0' &&
         strstr( (const char *) buf, "-----BEGIN CERTIFICATE-----" ) != NULL )
     {
         buf_format = MBEDTLS_X509_FORMAT_PEM;