Use internal images in template_icon_view

Question

I'm trying to fill a template_icon_view with Blender's default brushes, here's the code:

import bpy
import bpy.utils.previews
from bpy.types import WindowManager, EnumProperty
from bpy.props import EnumProperty

def getBrushPreviews(self, context):
    import bpy
    previews = []
    index = 0
    for brush in bpy.data.brushes:
        previews.append((brush.name, brush.name, "", brush.preview.icon_id, index))
        index += 1
    pcoll = preview_collections["main"]
    pcoll.brush_selection_previews = previews
    return pcoll.brush_selection_previews

class BrushSelectionPanel(bpy.types.Panel):
    """Creates a new panel for brush selection with categories."""
    bl_label = "Brush Selection"
    bl_idname = "brush_selection_panel"
    bl_category = "Advanced Brushes UI"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'TOOLS'

    def draw(self, context):
        layout = self.layout
        wm = context.window_manager

        row = layout.row()
        row.template_icon_view(wm, "brush_selection_previews")

        row = layout.row()
        row.prop(wm, "brush_selection_previews")

class BrushSelectionPanelReload(bpy.types.Operator):
    """Reload brush selection panel brushes."""
    bl_label = "Reload brush selection panel"
    bl_idname = "brush_selection_panel_reload"


#This is where the previews are stored
preview_collections = {}

def registerBrushSelectionPanel():
    WindowManager.brush_selection_previews = EnumProperty(
            items=getBrushPreviews,
            )
    pcoll = bpy.utils.previews.new()
    pcoll.brush_selection_previews = ()
    preview_collections["main"] = pcoll
    bpy.utils.register_class(BrushSelectionPanel)

It finds the brushes just fine, but doesn't actually show any preview images, just blank spaces where the images should be. Any ideas why?