R: Iterate over column with conditional index

Question

I have data from another program with poor formatting where each company's name is given once as a header, but then NA for each subsequent record.

company invoice  item
<chr>     <dbl> <dbl>
A            NA    NA
NA            1     5
NA            1     3
B            NA    NA
NA            2     2
C            NA    NA
NA            3     4
NA            4     7
NA            4     6
NA            5     8

I want to iterate through company replacing the NA values for each company with their name:

company invoice  item
<chr>     <dbl> <dbl>
A            NA    NA
A             1     5
A             1     3
B            NA    NA
B             2     2
C            NA    NA
C             3     4
C             4     7
C             4     6
C             5     8

Because the first instance of each company's name is correct, it made sense to me to use the index of company[[i - 1]] for each instance of NA. So, I was trying to iterate through the indexes with variations on the following:

df$company <- for (i in seq_along(df$company)) {
  if (is.na(df$company[[i]]) == TRUE) {
    df$company[[i]] = df$company[[i - 1]]
  } else {
    df$company[[i]] = df$company[[i]]
  }
}

Either it returns the df with company missing, or it returns NULL. What do I need to change to get the result I want?