Not a function TypeError when trying to split a user input inside a function

I created two functions and a global object data. I am trying to split what a person inputs inside a text area into individual strings and push them in the global object. The Error I have says userInputParam.split is not a function.

let data = {
  userInput: [],
  splittedInput: [],
  slicedInput: [],
};


function updateUserInput(data) {
  if (data.userInput.length == 0) {
    //   console.log("You can do this Panda!")
    data.userInput.push(input.value);
    //input.value is what a user inputs in a textarea. 
    splitUserInput(data.userInput)
  }
}

function splitUserInput (userInputParam){
    let splittedInput = userInputParam.split(/([*/+-])/)
    //console.log(splittedInput)
}