diff --git a/py/mpconfig.h b/py/mpconfig.h
index 63438a8465..b93f851d6c 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -383,6 +383,8 @@
 
 // Whether to use computed gotos in the VM, or a switch
 // Computed gotos are roughly 10% faster, and increase VM code size by a little
+// Note: enabling this will use the gcc-specific extensions of ranged designated
+// initialisers and addresses of labels, which are not part of the C99 standard.
 #ifndef MICROPY_OPT_COMPUTED_GOTO
 #define MICROPY_OPT_COMPUTED_GOTO (0)
 #endif