diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 193974bf83bd1aa9a0da12124f373c8df37b648b..123248c176700b549316326ada3a206ce0c52224 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -931,6 +931,7 @@ DerivationGoal::HookReply DerivationGoal::tryBuildHook()
     }
     
     /* parent */
+    pid.setSeparatePG(true);
     logPipe.writeSide.close();
     worker.childStarted(shared_from_this(),
         pid, singleton<set<int> >(logPipe.readSide), false);