diff --git a/src/box/opt_def.h b/src/box/opt_def.h
index bb4621438f1b2e712487053aeb51e81cb3caf456..ad2de4942e8193e33f9b36c6b7013b9d1bb89cc4 100644
--- a/src/box/opt_def.h
+++ b/src/box/opt_def.h
@@ -101,7 +101,9 @@ struct opt_def {
 
 	const char *enum_name;
 	int enum_size;
-	const char **enum_strs;
+	/** Enum key names. Passed to strnindex. */
+	const char *const *enum_strs;
+	/** Enum key count. Passed to strnindex. */
 	uint32_t enum_max;
 	/** MsgPack data decode callbacks. */
 	union {