diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 87e01a378..9497e7b3e 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2686,6 +2686,12 @@ void DerivationGoal::startBuilder()
 
         userNamespaceSync.readSide = -1;
 
+        /* Close the write side to prevent runChild() from hanging
+           reading from this. */
+        Finally cleanup([&]() {
+            userNamespaceSync.writeSide = -1;
+        });
+
         pid_t tmp;
         if (!string2Int<pid_t>(readLine(builderOut.readSide.get()), tmp)) abort();
         pid = tmp;
@@ -2712,7 +2718,6 @@ void DerivationGoal::startBuilder()
 
         /* Signal the builder that we've updated its user namespace. */
         writeFull(userNamespaceSync.writeSide.get(), "1");
-        userNamespaceSync.writeSide = -1;
 
     } else
 #endif