diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 19e97aed9..0be003da2 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -259,6 +259,8 @@ struct CmdFlakeCheck : FlakeCommand, MixJSON
 
     void run(nix::ref<nix::Store> store) override
     {
+        settings.readOnlyMode = !build;
+
         auto state = getEvalState();
         auto flake = resolveFlake();