diff --git a/library/x509_csr.c b/library/x509_csr.c
index 5ec1b86..ebf8897 100644
--- a/library/x509_csr.c
+++ b/library/x509_csr.c
@@ -276,7 +276,7 @@
     mbedtls_pem_init( &pem );
 
     /* Avoid calling mbedtls_pem_read_buffer() on non-null-terminated string */
-    if( buf[buflen - 1] != '\0' )
+    if( buflen == 0 || buf[buflen - 1] != '\0' )
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     else
         ret = mbedtls_pem_read_buffer( &pem,