diff --git a/nixos/modules/installer/cd-dvd/iso-image.nix b/nixos/modules/installer/cd-dvd/iso-image.nix
index 5fdee20f55b..c430048d659 100644
--- a/nixos/modules/installer/cd-dvd/iso-image.nix
+++ b/nixos/modules/installer/cd-dvd/iso-image.nix
@@ -283,7 +283,7 @@ let
     cat <<EOF > $out/EFI/boot/grub.cfg
 
     set with_fonts=false
-    set textmode=${boolToString !config.isoImage.graphicalGrub}
+    set textmode=${boolToString (!config.isoImage.graphicalGrub)}
     # If you want to use serial for "terminal_*" commands, you need to set one up:
     #   Example manual configuration:
     #    → serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1