tikz-uml sequence diagram “Extra \or” error

Question

I was compiling the following code which uses the tikz-uml package.

\documentclass{article}
%\usepackage{pgf}
%\usepackage{tikz}
\usepackage{tikz-uml}

\begin{document}
\begin{tikzpicture}
    \begin{umlseqdiag}
        \umlobject{a}
        \begin{umlcall}{a}{a}
            \begin{umlcall}{a}{a}           % line 11
            \end{umlcall}                   % line 12
        \end{umlcall}
    \end{umlseqdiag}
\end{tikzpicture}
\end{document}

It compiles on one machine without any problems. However, on another one, it gives an error. Both machines use Windows 7 64-bit, MikTeX 2.9 64-bit. Error message:

! Extra \or.
\ifthenelse #1->\toks@ {#1}\TE@repl \or
                                        \TE@or \TE@repl \and \TE@and \TE@rep...
l.12            \end{umlcall}
I'm ignoring this; it doesn't match any \if.

Anyone knows the reason for it and how to resolve it? I know it's due to an umlcall environment inside another one. It's actually enough to uncomment lines 11 and 12 to make the error disappear, but the result is not the same obiously.

The problem doesn't depend on including PGF or TikZ packages, nor on defining inputenc or fontenc. It doesn't depend on the fact that the a object calls itself neither.

EDIT 1: I was mistaken, the code that I tested and that worked on other machine was actually a bit different (precisely: everything inside the tikzpicture environment was a part of a bigger file).

The solution is to add \usepackage[language]{babel} in the preamble. But why does the documentation not say anything about it...?

EDIT 2: The wanted result is:

Answer 1

The solution is to use the babel package with the language of your choice, i.e. add one line to the preamble:

\usepackage[english]{babel}

Of course, you can change english to use different language.

The documentation of the tikz-uml package says nothing about this issue, nothing like "required packages" is mentioned there.