TikZ foreach Schleife - Warum wird Zahlenfolge nicht erkannt?

\documentclass{scrartcl}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[
%font=\footnotesize, 
>=latex, 
scale=0.65,
x=10cm, y=10cm, 
]

%KoSy
%Gitternetzlinien
%\draw[very thin,color=gray] (-2,-4) grid (3,4);
%Achsen
\draw[->] (-0.05,0) -- (1.075,0) node[right, below] {Re};
\draw[->] (0,-0.05) -- (0,1.075) node[above, left] {Im};
%Achsenbeschriftung
%Zahlen auf x-Achse
\foreach \x in {0.1,0.2,...,1} {
  \pgfkeys{/pgf/number format/.cd,fixed,fixed zerofill,precision=1}
  \draw (\x,2pt) -- (\x, -2pt) node[below, font=\footnotesize] {$\pgfmathprintnumber{\x}$};
}

%Zahlen auf y-Achse
\foreach \y in {0.1,0.2,...,1.0} {
  \pgfkeys{/pgf/number format/.cd,fixed,fixed zerofill,precision=1}
  \draw (2pt,\y) -- (-2pt,\y) node[left, font=\footnotesize]{$\pgfmathprintnumber{\y}$};
}

%Ursprung
\node[below left] {$0$};

%Funktionen
\def\a{0.3482759645}
\def\b{0.6517240354}

\coordinate(O) at (0,0);
\coordinate(Z) at (\a,\b);
\coordinate(W) at (\b,\a);
\coordinate(ZplusW) at (\a+\b,\b+\a);
\coordinate(absZi) at (0,\a^2+\b^2);

%z=a+ib
\draw[red, thick, ->] (O) -- (Z) node[sloped, above, midway, text=black, font=\sffamily]{(a1)} node[left]{$z$};
\end{tikzpicture}

\end{document}