Cannot add a new entry string field, "name already exists" error

I found myself not able to add new String Fields to Entries. Instead of creating a new field, an error is displayed.

Reproduction:

1. Install and run Keepass2
2. Create a new database
3. Create a new entry, or edit one of the default ones
4. On the "Edit Entry" window switch to the tab "Advanced", and click "Add" next to the "String fields" list
5. Enter any "Name", and "Value"
6. Try to save the field by clicking "OK"

What I expected:

Creation of a new entry in the "String fields" list

What happened:

After clicking "OK" a red text below the "Name" field apprears, telling me that "The entered name exists already and cannot be used"

Happens even in a brand new database, so what the error says is impossible.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: keepass2 2.18+dfsg-2
ProcVersionSignature: Ubuntu 3.2.0-24.37-generic-pae 3.2.14
Uname: Linux 3.2.0-24-generic-pae i686
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu7
Architecture: i386
Date: Sat Apr 28 16:47:58 2012
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, no user)
 LANG=pl_PL.UTF-8
 SHELL=/bin/bash
SourcePackage: keepass2
UpgradeStatus: No upgrade log present (probably fresh install)