If statement returns argument of length 0 error when called from a function, but works fine when called line by line

WARNING: tl;dr! So thank you in advance for your time.

I'm writing a set of functions to parse and process emails and I'm getting stuck on what seems like a trivial error. I have a set of functions that takes in a set of mbox files one by one, makes a corpus of the emails in that mbox file and then parses each email one by one to make a data frame. A snippet of the code follows:

library(tm)
library(tm.plugin.mail)

parseSingleMbox <- function(mailbox) {
  convert_mbox_eml(mailbox, '~/Temp/OSE/Temp')
  myCorpus <- Corpus(DirSource('~/Temp/OSE/Temp'), readerControl=list(reader=readMail))
  unlink('~/Temp/OSE/Temp', recursive=T)
  return(myCorpus)
}

parseEmail <- function(email) {
  #Content
  content <- email$content
  idx <- grep('^Content-Type: ', content)
  start <- idx - 1
  finish <- c(idx[-1]-2, length(content))

  for (i in 1:length(idx)) {
    #Get formatting data
    c1 <- content[start[i]:finish[i]]
    type <- grep('^Content-Type: ', c1, ignore.case=F, value=T) %>% 
      str_extract(., ': .*;') %>% 
      substr(., 3, nchar(.)-1)
    encoding <- grep('^Content-Transfer-Encoding: ', c1, ignore.case=F, 

    #Get body of email
    print(type)
    if (type == 'text/plain') {
      idxx <- which(nchar(c1) == 0) %>% min()
      c2 <- c1[idxx:length(c1)] %>% paste(sep='', collapse='')
      content <- formatEmail(...)
      body <- content
    }
  }
}

emailsToDF <- function(projectDir, pattern='.*\\.mbox$') {
  files <- list.files(path=file.path(projectDir, 'Originals'), pattern=pattern, full.names=T, recursive=T)
  dat <- NULL
  for (i in 1:length(files)) {
    mboxCorpus <- parseSingleMbox(files[i])
    for (j in 1:length(mboxCorpus)) {
      dat.temp <- parseEmail(mboxCorpus[[i]])
      dat <- rbind(dat, dat.temp)
    }
  }
  return(dat)
}

I don't know how to provide a reproducible example here because these emails are confidential, but here is a redacted version:

> email$content
 [1] "--0016e6de00577b89540497f2f3b3"                                              
 [2] "Content-Type: text/plain; charset=UTF-8"                                     
 [3] "Content-Transfer-Encoding: base64"                                           
 [4] ""                                                                            
 [5] "[:alnum:]"
 [6] "[:alnum:]"
 [7] "[:alnum:]"

The problem is that when I run this function I get an error back on the if (type == 'text/plain') line saying the argument is of length 0. If I run the code line by line, however, using the exact same email, it works fine. What am I missing?

Thanks again for your time!