diff --git a/src/libstore/build/local-derivation-goal.hh b/src/libstore/build/local-derivation-goal.hh
index 070ae53f3..f92280aa1 100644
--- a/src/libstore/build/local-derivation-goal.hh
+++ b/src/libstore/build/local-derivation-goal.hh
@@ -92,8 +92,8 @@ struct LocalDerivationGoal : public DerivationGoal
        result. */
     std::map<Path, ValidPathInfo> prevInfos;
 
-    uid_t sandboxUid() { return usingUserNamespace ? (buildUser->getUIDCount() == 1 ? 1000 : 0) : buildUser->getUID(); }
-    gid_t sandboxGid() { return usingUserNamespace ? (buildUser->getUIDCount() == 1 ? 100  : 0) : buildUser->getGID(); }
+    uid_t sandboxUid() { return usingUserNamespace ? (!buildUser || buildUser->getUIDCount() == 1 ? 1000 : 0) : buildUser->getUID(); }
+    gid_t sandboxGid() { return usingUserNamespace ? (!buildUser || buildUser->getUIDCount() == 1 ? 100  : 0) : buildUser->getGID(); }
 
     const static Path homeDir;