diff --git a/nixos/modules/leasereport.nix b/nixos/modules/leasereport.nix
index 095ad090f369d93c83683bb58786dd3eebb81208..d2d17294f0d623a0a180bf953e6984ca631c48da 100644
--- a/nixos/modules/leasereport.nix
+++ b/nixos/modules/leasereport.nix
@@ -7,6 +7,6 @@ let
   # is known to work with LeaseReport.  It could be bumped if necessary but
   # this would probably only happen as a result of bumping the resolver in
   # stack.yaml.
-  haskellNixSrc ? builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/31fd01e14729e686de0a915b9bc4ff70397a84c7.tar.gz
+  haskellNixSrc = builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/31fd01e14729e686de0a915b9bc4ff70397a84c7.tar.gz;
 in
   (callPackage "${leasereport}/nix" { inherit haskellNixSrc; }).LeaseReport.components.exes.LeaseReport