Adding elements to a dataframe in R using Rbind

I am creating a dataframe with 3 columns (char, char, int) called Alleles_df from df1 and df2 using:

Alleles_df <- data.frame('refsnp_id'=character(),'allele'=character(), 
   'chrom_start' = integer(),stringsAsFactors = F)

for (i in 1:nrow(df1)){    
   Alleles_df[i,] <- df1[i,][(df1$col1[i]==df2$col1[i]),]
}

for some values of i, I receive the following error:

Error in x[[jj]][iseq] <- vjj : replacement has length zero

This is because the the columns df1 and df2 do not match for certain values of i. How do I bind a row with c("NA","NA",0) in those situations? I would greatly appreciate your assistance!

df1 is data from an online server called biomart. df2 is what I generated manually. Each has 3 columns with Chromosome, Allele, BaseLocation.

    refsnp_id allele chrom_start
1 rs778598915  G/A/T    42693910
2  rs11541159    T/C    42693843
3 rs397514502    G/C    42693321
4 rs762949801    C/T    42693665
5 rs776304817  G/A/T    42693653