Empty query string arg causes form field to display a comma

[bluzcat]

Hi there. I have a case where a form gets redisplayed after a validation error. The URL contains a query argument that is empty. The field exists in the model as well, and so it should be bound automatically (I'm using the <form:form> JSTL stuff). The effect of the query string coming in empty is that the corresponding form field gets displayed with a comma in it. However, if I hit reload in the browser, it goes away. I'm puzzled.
Here's the JSP snippet:

<form:form commandName="configFormModel" target="${target}">
  <table border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr valign="bottom">
        <td><b>Enter a name for this search</b><br/>
          <form:input path="searchInstanceName" maxlength="128" size="35" /></td>
        <td><form:errors path="searchInstanceName" cssClass="errorBox" /></td>

and here's the corresponding stuff from the model. Nothing strange here.

  private String searchInstanceName = null;

  public String getSearchInstanceName()
  {
    return searchInstanceName;
  }
  
  public void setSearchInstanceName(String searchInstanceName)
  {
    this.searchInstanceName = searchInstanceName;
  }

The controller overrides processFormSubmission() as follows:

    if (errors.hasErrors())
    {
      BindException newErrors = getErrorsForNewForm(request);
      newErrors.addAllErrors(errors);
      
      return showForm(request, response, newErrors, request.getParameterMap());
    }
    
    return onSubmit(request, response, command, errors);

Any help in shedding some light on this is highly appreciated. Thanks.