FlashFileStream not accepts bracket notation with range

In the documentation of FlashFileStream class says:

The memory buffer can also be accessed via bracket notation. The following is valid syntax:
f = flash.FlashFileStream(0x0800000,512)
f[0] = 1
x = f[10:20]

But when I try to get the value using it, a runtime error occours: Thread 1 exited with exception UnsupportedError @[0045:001E:0000:0000:0000:0000:0000:0000]

My code derives from the Flash Internal example

import streams
import json
import flash

## Warning! This example works on the Particle Photon only!
# You need to change the flash address to use another board
#  --> For Sam3X based boards you can safely use 0xe0000

streams.serial()


print("create flash file")

# open a 512 bytes FlashFileStream at address 0x80E0000
ff = flash.FlashFileStream(0x00310000,512)

print("reading flash file")

for i in range(30):
    print(i,"->",str(ff[i]))

hh = {
    "type":"thing",
    "data":23.5
}

print("Dumping json: ", hh)
ds = json.dumps(hh)

# save length and json to flash
print("Writing to buffer")
ff.write(len(ds))
ff.write(ds)

hh = ff[4:32]
print("hh: ", hh)