w4k fails reading modbus rtu float32

[cgn]

I've a eastron sdm72d modbus meter and I use the following lua code: 

Code:

require('luamodbus')
mb = luamodbus.rtu()

mb:open('/dev/RS485', 9600, 'N', 8, 1, 'H')   -- SDM72 nutzt 8N1
mb:setslave(20)
mb:connect()


local r1, r2 = mb:readinputregisters(70, 2)
local value = mb:readregistervalue(70, "float32")
log(r1, r2, value)


mb:close()

I get the following result:

Code:

* arg: 1
  * number: 16967
* arg: 2
  * number: 57672
* arg: 3
  * number: 0

So in general the code seems to work. Why do I get a 0 for value? 
The modbus register can be found here: https://data.xn--stromzhler-v5a.eu/manua...72dmv2.pdf