diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 674b2eaa3..3aa85e264 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -861,6 +861,9 @@ void LocalDerivationGoal::startBuilder()
                             _exit(1);
                         if (!userNamespacesEnabled && errno==EPERM)
                             warn("user namespaces appear to be disabled; they are required for sandboxing; check /proc/sys/user/max_user_namespaces");
+                        Path procSelfNsUser = "/proc/self/ns/user";
+                        if (!pathExists(procSelfNsUser))
+                            warn("/proc/self/ns/user does not exist; your kernel was likely built without CONFIG_USER_NS=y, which is required for sandboxing");
                         /* Mention sandbox-fallback in the error message so the user
                            knows that having it disabled contributed to the
                            unrecoverability of this failure */