diff --git a/src/queue/IdTable.hxx b/src/queue/IdTable.hxx
index bcc1b0ea7..60e184091 100644
--- a/src/queue/IdTable.hxx
+++ b/src/queue/IdTable.hxx
@@ -34,7 +34,7 @@ class IdTable {
 
 	unsigned next;
 
-	int *data;
+	int *const data;
 
 public:
 	IdTable(unsigned _size) noexcept