The following biomaRt query returns the minor_allele column as logical instead of character:
 

library("biomaRt")

mart <- useMart(host="feb2014.archive.ensembl.org", biomart="ENSEMBL_MART_SNP")
ensembl <- useDataset("hsapiens_snp", mart=mart)


res <- getBM(attributes=c("chr_name","chrom_start","chrom_strand","allele","refsnp_id", "refsnp_source","minor_allele", "minor_allele_freq"),
filters=c("chr_name", "chrom_start", "chrom_end"), values=list("11", 108202735,108202740), mart=ensembl)

> res
  chr_name chrom_start chrom_strand        allele   refsnp_id refsnp_source
1       11   108202738            1           C/T rs186666661         dbSNP

  minor_allele minor_allele_freq
1         TRUE       0.000459137

> class(res$minor_allele)
[1] "logical"

If using a wider range the minor allele is returned correctly:

> res <- getBM(attributes=c("chr_name", "chrom_start","chrom_strand","allele","refsnp_id", "refsnp_source",  "minor_allele", "minor_allele_freq"),
filters=c("chr_name", "chrom_start", "chrom_end"), values=list("11", 108202650,108202740),
mart=ensembl)

> res
  chr_name chrom_start chrom_strand        allele   refsnp_id refsnp_source
1       11   108202681            1 HGMD_MUTATION    CD991612   HGMD-PUBLIC
2       11   108202681            1 HGMD_MUTATION    CD991613   HGMD-PUBLIC
3       11   108202685            1           A/G  rs28904920         dbSNP
4       11   108202685            1 HGMD_MUTATION   CM0910523   HGMD-PUBLIC
5       11   108202716            1           A/C rs199915459         dbSNP
6       11   108202738            1           C/T rs186666661         dbSNP
  minor_allele minor_allele_freq
1                             NA
2                             NA
3                             NA
4                             NA
5            C       0.000459137
6            T       0.000459137

> class(res$minor_allele)
[1] "character"

Is there any way to circumvent this behaviour

Regards,

Christian