In pgfplots, font=\small throws undefined control sequence

Question

I would like to reduce the label font size in a pgfplot.

When I do x tick label style={rotate=90, anchor=east}, the file compiles, when I do x tick label style={font=\small,rotate=90, anchor=east}, to reduce the font size, I get

! Undefined control sequence.
\tikz@textfont ->\small 

l.14 \end{axis}
               \end{tikzpicture}
? 

Where is the problem? I've seen this expression working in hundreds of examples on the web.

---

Full code:

\documentclass{minimal}

\usepackage{tikz}
\usepackage{pgfplots}
\usepackage[scaled]{helvet}
\usepackage[T1]{fontenc}
\usepackage{verbatim}

\renewcommand*\familydefault{\sfdefault} %% Only if the base font of the document is to be sans serif

\usetikzlibrary{plotmarks}

%%%<
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{tikzpicture}
\PreviewEnvironment{verbatim}
\setlength\PreviewBorder{5pt}
%%%>

\begin{document}

\begin{tikzpicture}\begin{axis}[
title = { Title },
ymin=0, xbar, bar width=10cm /  67.5 , width=10cm, xtick={1,..., 27 },
xticklabels={%
Sample 1 ,
Sample 2,
Sample 3,
}, x tick label style={font=\small,rotate=90, anchor=east}, ylabel={Concentration [pg / $10^5$ cells]},]
\addplot[fill=black,draw=black,ybar,error bars/.cd,y dir=both,y explicit] coordinates {
( 1 , 0.0 ) +- ( 0.0 , 0.0 )
( 2 , 3.38661354582 ) +- ( 4.78939480701 , 4.78939480701 )
( 3 , 1.05696969697 ) +- ( 1.49478088047 , 1.49478088047 )
};
\end{axis}\end{tikzpicture}

\end{document}