diff --git a/src/libstore/include/nix/store/ssh.hh b/src/libstore/include/nix/store/ssh.hh
index 7e27a0d3e..574cb5cf4 100644
--- a/src/libstore/include/nix/store/ssh.hh
+++ b/src/libstore/include/nix/store/ssh.hh
@@ -27,7 +27,7 @@ private:
     const bool compress;
     const Descriptor logFD;
 
-    ref<AutoDelete> tmpDir;
+    const ref<const AutoDelete> tmpDir;
 
     struct State
     {