custom blender import addon, filter_glob could not register

I’ve written a small addon to import models that have been saved by VTK(C++) in ASCII format

bl_info = {#represents data about the script and the version of blender it is intended for#and where it exists in the menu structure
    "name":            "Import VTK",
    "author":        "Sum Yung Gai",
    "blender":        (2,68,0),
    "version":        (1,0,0),
    "location":        "File > Import-Export",
    "description":     "Import VTK PolyData(.vtk)",
    "category":        "Import-Export"
}
import bpy #import bpy for blender python API access
from bpy.props import *
import os #import os for filename and directory manipulation
import re #import re for regular expression manipulation
from bpy_extras.io_utils import ImportHelper
from bpy.props import (StringProperty,
                       BoolProperty,
                       CollectionProperty,
                       EnumProperty,
                       FloatProperty,
                       )
from bpy_extras.io_utils import (ImportHelper)
from bpy.types import Operator, OperatorFileListElement


class ImportVTK(bpy.types.Operator, ImportHelper):
    """Load VTK triangle mesh data"""
    bl_idname = "import_mesh.vtk"
    bl_label = "Import VTK mesh"
    bl_options = {'UNDO'}
    filename_ext = ".vtk"
    filter_glob = StringProperty(name=".vtk",default="*.vtk", options=('HIDDEN'))
    files = CollectionProperty(name="File Path",type=OperatorFileListElement,)
    directory = StringProperty(subtype='DIR_PATH',)
    def execute(self, context):
        from . import blender_utils
        paths = [os.path.join(self.directory, name.name)
                 for name in self.files]
        if not paths:
            paths.append(self.filepath)
        if bpy.ops.object.mode_set.poll():
            bpy.ops.object.mode_set(mode='OBJECT')
        if bpy.ops.object.select_all.poll():
            bpy.ops.object.select_all(action='DESELECT')
        for path in paths:
            objName = bpy.path.display_name(os.path.basename(path))
            importVTK(path)
        return {'FINISHED'}

Every time I try to register this, blender prints the following to the system console:

TypeError: StringProperty() argument 6 must be set, not str
ValueError: bpy_struct "IMPORT_MESH_OT_vtk" registration error: filter_glob could not register

I did copy this code out of the STL import/export addon and then did a bit of surgery to make it what I wanted. Apparently I missed some key bits when I was doing it. Any suggestions/ideas are appreciated.

The options needs to be in curly braces


options = {'HIDDEN'}