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

R doesn't like me today...

I have two tables asembled via cbind(). Tab 1 (dwd_nogap) is

  x1                 col1_x1       col2_x1      
A "1982 12 01 00:00""        0.4""          0"
B "1982 12 02 00:00""       -0.5""          0"
C "1982 12 03 00:00""       -0.2""          0"
D "1982 12 04 00:00""         -1""        0.1"
E "1982 12 05 00:00""       -0.9""          0"
F "1982 12 06 00:00""        3.7""        4.1"

Tab 2 (dwd_gap) is:

     x2                 col1_x2       col2_x2      
[1,] "1982 12 01 00:00""        0.4""          0"
[2,] "1982 12 03 00:00""       -0.2""          0"
[3,] "1982 12 04 00:00""         -1""        0.1"
[4,] "1982 12 05 00:00""       -0.9""          0"
[5,] "1982 12 06 00:00""        3.7""        4.1"
[6,] "1982 12 07 00:00""          7""        5.8"

My merge command is:

exporttab <- merge(x=dwd_nogap,y=dwd_gap,by.x=dwd_nogap[,1],by.y=dwd_gap[,1], fill=-9999)

In my opinion the command is correct, but it's apparently not doing well...

Error in fix.by(by.x, x) : 'by' must specify uniquely valid columns