Message error with tkz-fct

I have a problem with the tkz-fct package. I now have this error message when compiling:

! Undefined control sequence. <argument> \tkz@tmp@xa
l.15 \tkzFct{\x}
?

This is the code:

\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[left=1.5cm,right=1.5cm,top=2cm,bottom=2cm]{geometry}
\usepackage{amsmath,amsfonts,amssymb}
\usepackage{tikz,tkz-tab}
\usepackage{tkz-fct}

\begin{document}

\begin{tikzpicture}[scale=1.3]
\tkzInit[xmin=-1,xmax=4,ymin=0,ymax=5]
\tkzGrid
\tkzAxeXY
\tkzFct[domain=0:4]{\x}
\tkzFct[domain=0:4]{\x-1}
\tkzDrawAreafg[color=green,domain = 1:3]
\tkzRep
\tkzText(5.2,3.5){$\displaystyle{\int_{1}^3 f(x)-g(x)dx}$}
\draw [->] (4.1,3.5) -- (2.6,3);
\tkzText(2.5,4.2){$\mathscr{C}_f$}
\tkzText(2.5,1.2){$\mathscr{C}_g$}
\end{tikzpicture}   
\end{document} 

I am using the last version of TikZ and of tkz-fct. The PDF file is compiling but I have this (these) error(s) anyway.

Can someone tell me where it comes from?

Tried with the suggested mods but I have the same message. The problem seems to come from the tkz-fct package. I use the last version of tkz-fct.sty