Error in data frame arguments imply differing number of rows

Here is my code:

assign2.aggregData <- data.frame(subject.ID = assign2.rawData$subject.ID,
                                 age = assign2.rawData$age,
                                 meanRT_dom,
                                 meanRT_nondom)

However, the number of rows in my data is equal to the number of columns so I am not sure how to fix the error:

Error in data.frame(assign2.rawData$subject.ID, age = assign2.rawData$age, : arguments imply differing number of rows: 0, 16