Why selectCheckboxMenu passes values as its toString?

Question

<p:selectCheckboxMenu id="producers"
                      value="#{products.producers}"
                      label="producers"
                      filter="true"
                      filterMatchMode="contains"
                      panelStyle="width: 400px"
                      style="width: 400px">
  <p:ajax update="productsTable"/>
  <f:selectItems value="#{producers.producers}" var="producer"
                 itemLabel="#{producer.name} (#{producer.uri})"/>
</p:selectCheckboxMenu>

My Product has following method.

public void setProducers(final List<Producer> producers) {
}

The list doesn't contains Producers but the String value of toString. What did I do wrong?