diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index 228871d2525..98aaee00bfd 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -872,23 +872,29 @@ ShmemAddrIsValid(const void *addr)
 void
 RegisterShmemCallbacks(const ShmemCallbacks *callbacks)
 {
-	if (shmem_request_state == SRS_DONE && IsUnderPostmaster)
+	if (shmem_request_state == SRS_DONE)
 	{
 		/*
 		 * After-startup initialization or attachment.  Call the appropriate
 		 * callbacks immediately.
+		 *
+		 * This is not allowed from the postmaster, because the postmaster
+		 * cannot acquire locks.
 		 */
 		if ((callbacks->flags & SHMEM_CALLBACKS_ALLOW_AFTER_STARTUP) == 0)
 			elog(ERROR, "cannot request shared memory at this time");
+		Assert(IsUnderPostmaster || !IsPostmasterEnvironment);
 
 		CallShmemCallbacksAfterStartup(callbacks);
 	}
-	else
+	else if (shmem_request_state == SRS_INITIAL)
 	{
 		/* Remember the callbacks for later */
 		registered_shmem_callbacks = lappend(registered_shmem_callbacks,
 											 (void *) callbacks);
 	}
+	else
+		elog(ERROR, "cannot request shared memory at this time");
 }
 
 /*
diff --git a/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl b/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
index 5cf07d071ec..546d6a92abe 100644
--- a/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
+++ b/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
@@ -27,6 +27,28 @@ cmp_ok($attach_count2, '>', $attach_count1,
 	"attach callback is called in each backend");
 $node->stop;
 
+###
+# Test allocating memory after startup in single-user mode
+###
+SKIP:
+{
+	# Skip the test on Windows, as single-user mode would fail on permission
+	# failure with privileged accounts.
+	skip 'single-user test is not supported by this platform', 1
+	  if $windows_os;
+	my $query = "SELECT get_test_shmem_attach_count();\n";
+	my $result = run_log(
+		[
+			'postgres', '--single', '-F',
+			'-c' => 'exit_on_error=true',
+			'-D' => $node->data_dir,
+			'postgres'
+		],
+		'<' => \$query);
+
+	ok($result, "shmem area is initialized in single-user mode");
+}
+
 ###
 # Test that loading via shared_preload_libraries also works
 ###