Is there any limitation on the name of the properties of contacts?

[#1]

Hi,
sometime when calling the add contact endpoint with some custom properties some field are ignored.
So the question is, is there a limitation on the names like, for example, name containing _ or ? or . $?
for example a property with the following name is ok?
what_crm_do_you_use_?

thanks!

[Derek_Gervais]

Hi @pie,

Property names must contain only lowercase letters, numbers, and underscores, and they must start with a letter. If you attempt to create a property with an invalid name, you’ll see the above in the resulting error message.