diff --git a/src/libstore/normalise.cc b/src/libstore/normalise.cc
index f4159f65a1..bf096bde7f 100644
--- a/src/libstore/normalise.cc
+++ b/src/libstore/normalise.cc
@@ -1441,6 +1441,7 @@ Worker::Worker()
     /* Debugging: prevent recursive workers. */ 
     if (working) abort();
     working = true;
+    nrChildren = 0;
 }