Error in if (prior.scale <= 0) { : argument is of length zero - add_regressor() function in R

Hi. Im fairly new to using fbprophet and have encountered an error while trying to add regressors to fit my model. I have 8 regressors, all of which are numeric. Tried fitting the regressors using the code :
`modelProphet <- add_regressor(pcedataProphet_train, 'PC1')`

pcedataProphet_train- df
PC1 - regressor

I have attempted to add all 8 regressors using the above line of code. But the error that is shown is:
`Error in if (prior.scale <= 0) { : argument is of length zero`.

I checked on what this means but did not understand what value should I add to my argument 'prior.scale'.

Thanks in Advance!