What is the correct CRC value for a zero length check?

I have a CRC-32/ISO-HDLC calculation function which is coded in 'C'. It works fine in that if I feed it '123456789' it give the correct result 0xCBF43926. However what is the correct result I should get for the empty string '' ?

My function gives '0x00000000' which is understandable given the way the code is written. But the reference calculator I am checking against gives 0xD202EF8D !!

I am checking against this tool : https://crccalc.com/?crc=&method=crc32&datatype=ascii&outtype=hex

My code is based on this reference : https://tools.ietf.org/html/rfc1662 which uses a lookup table technique.

Some other online tools also give 0x0000 for zero length input. Which tool is correct on this subject ?

I am checking against this tool : https://crccalc.com/?crc=&method=crc32&datatype=ascii&outtype=hex

I am not convinced that this tool works for zero lengths. A value of 0x00000000 is correct for a zero length.

It does works correctly for me with all kinds of other data lengths and data.