Why is the optional block starting out unchecked, but expanded?

config.jelly

<f:optionalBlock name="generatingParcelBuilderInputFile"  checked="${instance.getParcelBuilderInputFilename() != null}"
                     title="Generate input file for ParcelBuilder?">
    <f:entry title="Filename" field="parcelBuilderInputFilename">
        <f:textbox/>
    </f:entry>
</f:optionalBlock>

StoreSCM.java

public class StoreSCM extends SCM {
    private String parcelBuilderInputFilename;

    @DataBoundConstructor
    public StoreSCM(String parcelBuilderInputFilename) {
        this.parcelBuilderInputFilename = parcelBuilderInputFilename;
    }

    public String getParcelBuilderInputFilename() {
        return parcelBuilderInputFilename;
    }

//...
    @Override
    public DescriptorImpl getDescriptor() {
        return (DescriptorImpl)super.getDescriptor();
    }

    @Extension
    public static final class DescriptorImpl extends SCMDescriptor<StoreSCM> {
        public DescriptorImpl() {
            super(StoreSCM.class, null);
            load();
        }

        @Override
        public String getDisplayName() {
            return "Store";
        }

        @Override
        public boolean configure(StaplerRequest request, JSONObject formData) throws FormException {
            request.bindJSON(this, formData);
            save();
            return true;
        }

        @Override
        public StoreSCM newInstance(StaplerRequest request, JSONObject formData) throws FormException {
            String parcelBuilderInputFilename = null;
            Object parcelBuilderOptions = formData.get("generatingParcelBuilderInputFile");
            if (parcelBuilderOptions != null) {
                parcelBuilderInputFilename = ((JSONObject) parcelBuilderOptions).getString("parcelBuilderInputFilename");
            }

            return new StoreSCM(parcelBuilderInputFilename);
        }
}