Bioconductor package "oligo" works fine on its own, but returns an error message when knitted as part of a .Rmd document

Question

I am using Bioconductor package "oligo" to read in and analyze some .cel files and am running into trouble, specifically with reading in the files. However, this error only occurs when the code is part of an R Markdown document, but not when I run it from RStudio, so I know there is no issue with the actual files.

Here are the contents of the .Rmd file:

---
title: "Untitled"
output: html_document
---

```{r}
library(oligo)
celFiles <- list.celfiles("./cel_files/", full.names=TRUE)
raw.data <- read.celfiles(celFiles)
```

Knitting this file gives the following output:

Quitting from lines 12-15 (DE_Analysis.Rmd) 
Error: checkChipTypes(filenames, verbose, "affymetrix", TRUE) is not TRUE
Execution halted

However, running the following code in Rstudio successfully reads in the files:

library(oligo)
celFiles <- list.celfiles("./cel_files/", full.names=TRUE)
raw.data <- read.celfiles(celFiles)

It seems that something about R Markdown does not agree with oligo -- any ideas as to what might be happening and how to fix the problem?