diff --git a/zkapauthorizer.nix b/zkapauthorizer.nix
index bb7e5e21b46b1ab8d47e64bee5522cd01744176f..3edef94512311e56a8399cfe9f1c796cf547c839 100644
--- a/zkapauthorizer.nix
+++ b/zkapauthorizer.nix
@@ -10,7 +10,9 @@
 let
   hypothesisProfile' = if hypothesisProfile == null then "default" else hypothesisProfile;
   testSuite' = if testSuite == null then "_zkapauthorizer" else testSuite;
-  extraTrialArgs = builtins.concatStringsSep " " (if trialArgs == null then ["--rterrors" "--jobs=4" ] else trialArgs);
+  defaultTrialArgs = [ "--rterrors" ] ++ ( lib.optional ( ! collectCoverage ) "--jobs=4" );
+  trialArgs' = if trialArgs == null then defaultTrialArgs else trialArgs;
+  extraTrialArgs = builtins.concatStringsSep " " trialArgs';
 in
 buildPythonPackage rec {
   version = "0.0";