diff --git a/flake.nix b/flake.nix
index a274254..e990dbf 100644
--- a/flake.nix
+++ b/flake.nix
@@ -43,7 +43,7 @@
                   prootPatched
                 ];
               };
-              appBinGuess = "${drv}/bin";
+              mainProgram = drv.meta.mainProgram or (throw "${drv.name} has no meta.mainProgram set");
               prootStoreBase = builtins.baseNameOf prootPatched;
               PROOT_REL = "/nix/store/${prootStoreBase}/bin/proot";
             in
@@ -68,7 +68,7 @@
                 done < ${ci}/store-paths
 
                 # 2) pick app binary and make relative path used by the runner
-                APP_BIN=$(find ${appBinGuess} -maxdepth 1 -type f -perm -111 | head -n1)
+                APP_BIN="${drv}/bin/${mainProgram}"
                 if [ -z "''${APP_BIN:-}" ]; then
                   echo "no executable found in ${appBinGuess}" >&2
                   exit 1