INVALID_CUSTOM_TOKEN : Invalid assertion format. 3 dot separated segments required

A global variable is added as:
firebaseToken



and another service with the body:
{
“token”: “${GlobalVariable.firebaseToken}”,
"returnSecureToken: true,
}

the following message returns to me:

“error”: {
“code”: 400,
“message”: “INVALID_CUSTOM_TOKEN : Invalid assertion format. 3 dot separated segments required.”,
“errors”: [
{
“message”: “INVALID_CUSTOM_TOKEN : Invalid assertion format. 3 dot separated segments required.”,
“domain”: “global”,
“reason”: “invalid”
}
]
}
}