Grid exception

I am getting an exception when I execute the following line:
grid.setContainerDataSource(userContainer);

The exception contains following repeated text:
at java.lang.StringBuilder.append(Unknown Source)
    at com.vaadin.data.util.PropertysetItem.toString(PropertysetItem.java:160)
    at java.lang.String.valueOf(Unknown Source)

Previously I encountered this error when the bean added to the container had attributes that could not be rendered as a string. I removed these attributes from the container as follows:
userContainer.removeContainerProperty("companiesAdministrated");

However in this case I have used the removeContainerProperty method to remove everything I can think of and still have the error...

All suggestions welcome.

Kind regards

Sean