diff --git a/.circleci/config.yml b/.circleci/config.yml
index 22fd5b687586f9dce7d8a806f1b90f4a917a9469..8260202db8f093fef7ea918eba30a8e6f135391d 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -61,7 +61,7 @@ jobs:
           name: "Build System Test Driver"
           command: |
             export NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/$(cat nixpkgs.rev).tar.gz
-            nix-build --max-jobs 1 nixos/system-tests.nix -A driver
+            nix-build --max-jobs 1 --cores 1 nixos/system-tests.nix -A driver
 
           # Give it a good long while.  PaymentServer and its dependencies, in
           # particular, can take a while to build.