JSF ERROR - The value of columns attribute must be greater than zero

I'm having this problem, specifically in the SelectManyCheckBox component.

Follow my Bean code:

package controle;

import java.io.Serializable; import java.util.List;

import javax.faces.bean.ManagedBean; import javax.faces.bean.ViewScoped;

import excecoes.NegocioException; import model. ; import service. ; import util.Messages;

@ManagedBean @ViewScoped public class CadastroAtendimentoBean implements Serializable {

private static final long serialVersionUID = 1L;

private AtendimentoService atdService = new AtendimentoService();
private Atendimento atdSelecionado;
private Long idAtendimento;
private List<Pessoa> clientes;
private List<Servico> servicos;
private PessoaService pesService = new PessoaService();
private ServicoService serService = new ServicoService();
private Long indice;

public AtendimentoService getAtdService() {
    return atdService;
}


public void setAtdService(AtendimentoService atdService) {
    this.atdService = atdService;
}

public Atendimento getAtdSelecionado() {
    return atdSelecionado;
}

public void setAtdelecionado(Atendimento atdSelecionado) {
    this.atdSelecionado = atdSelecionado;
}

public Long getIdAtendimento() {
    return idAtendimento;
}

public void setIdAtendimento(Long idAtendimento) {
    this.idAtendimento = idAtendimento;
}


public List<Servico> getServicos() {
    return servicos;
}

public void setServicos(List<Servico> servicos) {
    this.servicos = servicos;
}


public PessoaService getPesService() {
    return pesService;
}

public void setPesService(PessoaService pesService) {
    this.pesService = pesService;
}

public List <Pessoa> getClientes() {
    return clientes;
}


public void setClientes(List <Pessoa> clientes) {
    this.clientes = clientes;
}


public ServicoService getSerService() {
    return serService;
}

public void setSerService(ServicoService serService) {
    this.serService = serService;
}

public Long getIndice() {
    return indice;
}

public void setIndice(Long indice) {
    this.indice = indice;
}

public void inicializar() {
    clientes = pesService.buscarTodos();
    setServicos(serService.buscarTodos());

    if(this.getIdAtendimento() != null) {
        this.atdSelecionado = this.atdService.buscaPorId(this.idAtendimento);
    }

    if(this.atdSelecionado == null) {
        this.atdSelecionado = new Atendimento();
    }

}

public void salvar() throws NegocioException {
    if (atdSelecionado != null){
        try {
            this.atdService.salvar(this.atdSelecionado);
        }catch(NegocioException e){
            Mensagens.msgErro("Erro ao salvar", e.getMessage());
        }
    }else{
        System.out.println("Cliente inválido!");
    }
}

public boolean isEditing() {
    return (atdSelecionado.getIdAntendimento() == null ? false : true);
}

}

The xhtml:

<ui:composition xmlns="http://www.w3.org/1999/xhtml"
xmlns:h="http://xmlns.jcp.org/jsf/html"
xmlns:f="http://xmlns.jcp.org/jsf/core"
xmlns:p="http://primefaces.org/ui"
xmlns:ui="http://xmlns.jcp.org/jsf/facelets"
xmlns:o="http://omnifaces.org/ui"
template="/WEB-INF/templates/Layout.xhtml">

<ui:define name="titulo">
    Atendimentos
</ui:define>

<ui:define name="conteudo">

 <p:toolbar>  
    <p:toolbarGroup>
       <p:commandButton value="Salvar" action="#{cadastroAtendimentoBean.salvar}" 
                          update="@form"/>
    </p:toolbarGroup>     


    <p:toolbarGroup align="right">
       <p:button value="Pesquisa" outcome="/pessoa/pesquisaPessoa.xhtml" />
    </p:toolbarGroup>    
 </p:toolbar>   



 <p:panelGrid  columns="2"  style="margin-bottom:10px" >

     <p:outputLabel for="id-data" value="Data" />
     <p:inputText   id="id-data" value="#{cadastroAtendimentoBean.atdSelecionado.dataAtendimento}"
                    required="true"  requiredMessage="Informe a data do atendimento"/>

     <p:outputLabel for="id-valor" value="Valor" />
     <p:inputText   id="id-valor" value="#{cadastroAtendimentoBean.atdSelecionado.valorAtendimento}"
                    required="true"  requiredMessage="Informe o valor do atendimento."/>

     <p:outputLabel for="id-hrain" value="Hora início" />
     <p:inputText   id="id-hrain" value="#{cadastroAtendimentoBean.atdSelecionado.horaInAtendimento}"
                    required="true"  requiredMessage="Informe a hora de início do atendimento."/>

     <p:outputLabel for="id-hrout" value="Hora fim" />
     <p:inputNumber id="id-hrout" value="#{cadastroAtendimentoBean.atdSelecionado.horaOutAtendimento}"
                     required="true" requiredMessage="Informe a hora do final do atendimento" /> 

     <p:outputLabel for="id-cliAtendimento" value="Cliente" />
     <p:selectOneMenu id="id-cliAtendimento"  value="#{cadastroAtendimentoBean.atdSelecionado.pessoaAtendimento}"
           required="true" requiredMessage="Cliente é obrigatório">
         <f:selectItem itemLabel="Selecione o cliente"  />
         <f:selectItems value="#{cadastroAtendimentoBean.clientes}" 
                        var="cli" 
                        itemLabel="#{cli.nomePessoa}" 
                        itemValue="#{cli}" />
     </p:selectOneMenu>

     <!-- ==================ERRO AQUI================ -->  
     <p:outputLabel for="id-servico" value="Servicos" />
     <p:selectManyCheckbox id="id-servico"
                           value="#{cadastroAtendimentoBean.atdSelecionado.servicos}"
                           converter="servicoConverter" layout="grid">
         <f:attribute name="collectionType" value="java.util.ArrayList"/>
         <f:selectItems value="#{cadastroAtendimentoBean.servicos}" 
                        var="servico" 
                        itemLabel="#{servico.descricaoServico}" 
                        itemValue="#{servico}" />

     </p:selectManyCheckbox>

</p:panelGrid>

These are the main JSF error outputs:

 Error Rendering View[/atendimento/cadastroAtendimento.xhtml]
 Caused by: javax.faces.FacesException: The value of columns attribute must be greater than zero.
 FATAL: JSF1073: javax.faces.FacesException obtido durante o             processamento de RENDER_RESPONSE 6: UIComponent-ClientId=, Message=The value of columns attribute must be greater than zero.