diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index f09437c6b3..e7a8ee434a 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1806,9 +1806,10 @@ void DerivationGoal::startBuilder()
     {
         pid = fork();
         if (pid == 0) initChild();
-        else if (pid == -1) throw SysError("unable to fork");
     }
 
+    if (pid == -1) throw SysError("unable to fork");
+
     /* parent */
     pid.setSeparatePG(true);
     builderOut.writeSide.close();