MERGE issues

#1
I've run this code (almost exactly) before and have not had issues but now I keep getting an error saying:

ERROR IN FIX.BY(BY.X,X) : 'BY' MUST SPECIFY A UNIQUELY VALID COLUMN.

I have one data set (myDATA) that has the following columns:
ROW ID DATE X

I have another data set (myDATA2) that has the following columns:
ROW2 Y

I'm trying to merge them by ROW = ROW2 and have the following code:

DATA_combo <- merge(myDATA, myDATA2, by.x="ROW", by.y="ROW2", all=FALSE)

But that does not work. Is something wrong with this code? I've done this exact code multiple times with other data sets and have had no issues so I don't understand what is wrong now.