Subcaption compatibility with ACM document class

I'm currently working on a paper for an ACM conference and they require me to use the sigplanconf.cls

I want to use subcaption, but if I add

\usepackage{caption}
\usepackage{subcaption}


then I get the following messages :

Package caption Warning: Unsupported document class (or package) detected,
(caption)                usage of the caption package is not recommended.
[...]
! Package caption Error: No float type 'copyrightbox' defined.


Anyone encounter that problem ? Is there a fix that doesn't require messing with the cls since I'm supposed to use it as is ?

Full MWE :

\documentclass[10pt]{sigplanconf}

\usepackage{amsmath}

\usepackage{caption}
\usepackage{subcaption}

\begin{document}

\conferenceinfo{XXX '13}{Date, Place.}

\title{Title Text}
\subtitle{Subtitle Text, if any}

\authorinfo{Name1}
{Affiliation1}
{Email1}

\maketitle

test

\end{document}

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I don't get any error when I load figures. About the warning message, ask them directly about the rules on using subfigures. –  hpesoj626 Dec 12 '12 at 2:30
I get a warning but no no error. In general, it means that the given documentclass has its own way of defining the caption command –  cmhughes Dec 12 '12 at 4:30
Is it possible taht it comes from my version of latex then ? –  Lexiel Dec 13 '12 at 17:57

The package caption is incompatible with the sigplanconf class that implements its own method for captions.

If you need to use the class, there's no way to incorporate subcaption that relies on caption working.

-
  \usepackage{caption}