"Incorrect number of dimensions" error when accessing column in .Rda in R Studio? -

this seems overly simplistic, can't beyond error "incorrect number of dimensions," when using following code access 1 column. typically work .csv files, i'm not sure if there's special need .rda file. .rda file contains 15 columns , on 100,000 rows. why mydata[,1] not work return first column of data?

mydata <- load('my_file_path.rda' )  mydata[,1]