Augment data frame missed values by another data frame

I have following data frames:

> df1 = data.frame(ind = 1:4, x=c('a', 'b', NA, 'd'))
 > df2 = data.frame(ind = 1:4, x=c(NA, NA, 'c', NA))
 > df1
   ind    x
 1   1    a
 2   2    b
 3   3 <NA>
 4   4    d
 > df2
   ind    x
 1   1 <NA>
 2   2 <NA>
 3   3    c
 4   4 <NA>
 

I want combine them filling missing values in df1 by numeric values from df2. How can I do that? I cannot do that neither with merge nor with join commands:

> merge(df1, df2, by='ind', all=T)
   ind  x.x  x.y
 1   1    a <NA>
 2   2    b <NA>
 3   3 <NA>    c
 4   4    d <NA>
 

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