diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index ac23cff..d725e29 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -98,7 +98,7 @@
  * Build flag from an algorithm/curve identifier (pk, md, ecp)
  * Since 0 is always XXX_NONE, ignore it.
  */
-#define MBEDTLS_X509_ID_FLAG( id )   ( 1 << ( id - 1 ) )
+#define MBEDTLS_X509_ID_FLAG( id )   ( 1 << ( ( id ) - 1 ) )
 
 /**
  * Security profile for certificate verification.