--- a/util/datamanager.lua	Thu Feb 25 22:33:58 2016 +0100
+++ b/util/datamanager.lua	Thu Feb 25 22:37:41 2016 +0100
@@ -152,6 +152,7 @@
 		if not ok then break end
 
 		ok, msg = f:close();
+		f = nil; -- no longer valid
 		if not ok then break end
 
 		return os_rename(scratch, filename);