diff --git a/pkgs/os-specific/linux/kernel/manual-config.nix b/pkgs/os-specific/linux/kernel/manual-config.nix
index b840e252cdf..acc59303559 100644
--- a/pkgs/os-specific/linux/kernel/manual-config.nix
+++ b/pkgs/os-specific/linux/kernel/manual-config.nix
@@ -104,7 +104,8 @@ let
         buildFlagsArray+=("KBUILD_BUILD_TIMESTAMP=Thu Jan 1 00:00:01 UTC 1970")
       '';
 
-      KBUILD_BUILD_TIMESTAMP=1; # (time_t)1
+      buildFlags = [
+        "KBUILD_BUILD_TIMESTAMP=1" # (time_t)1
         "KBUILD_BUILD_VERSION=1-NixOS"
         platform.kernelTarget
       ] ++ optional isModular "modules";