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)

Hi @JanainaScal
I am sorry for the late reply, Could you tell me which device have you used your code on?
Are you sure that the flash section you are trying to modify is reserved for the user?