I think I found the cause:

SendTo calls Bytes method on packet (a osc message)
https://github.com/scgolang/osc/blob/master/udp.go#L106C29-L106C29

Bytes method of message calls Bytes method for arguments, in this case a String
https://github.com/scgolang/osc/blob/master/message.go#L58

https://github.com/scgolang/osc/blob/master/argument.go#L239

Bytes method of String calls ToBytes. ToBytes returns []byte{} when len(string) == 0
https://github.com/scgolang/osc/blob/master/osc.go#L46

Then Pad doesn't append any null character (it isn't called)
https://github.com/scgolang/osc/blob/master/osc.go#L55

ToBytes is also used for msg.Address and BundleTag :
https://github.com/scgolang/osc/blob/master/message.go#L54
https://github.com/scgolang/osc/blob/master/bundle.go#L79

Both can't be empty according to the OSC specs I think*.

And a empty string is tested:
https://github.com/scgolang/osc/blob/master/osc_test.go#L14
It expects: []byte{}
but in this case it should be the following I think:
[]byte{0,0,0,0}

From the specs: osc string = A sequence of non-null ASCII characters followed by a null, followed by 0-3 additional null characters

(
Not related to this issue I think, but ReadString has a similar way of handling empty strings
https://github.com/scgolang/osc/blob/master/osc.go#L71
)

Looking at the gosc library, it checks if a string has "\x00" as suffix and adds it when it doesn't have any, as is the case with a empty string.
https://github.com/loffa/gosc/blob/master/writer.go#L104

I think something similar is done in the go-osc library:
https://github.com/hypebeast/go-osc/blob/master/osc/osc.go#L1027

• https://github.com/scgolang/osc/blob/master/bundle.go#L13
• An OSC Address Pattern is an OSC-string beginning with the character ‘/’ (forward slash).
  https://opensoundcontrol.stanford.edu/spec-1_0.html