[r] Merge r brings error "'by' must specify uniquely valid columns"

This is what I tried for a right outer join [as per my requirement]:

m1 <- merge(x=companies, y=rounds2, by.x=companies$permalink, 
            by.y=rounds2$company_permalink, all.y=TRUE)
# Error in fix.by(by.x, x) : 'by' must specify uniquely valid columns
m1 <- merge(x=companies, y=rounds2, by.x=c("permalink"), 
            by.y=c("company_permalink"), all.y=TRUE)

This worked.