isIncomplete(con) error when knitting pdf - rstudio

Question

I am able to run the following example script from the SWSamp package fine in the console, but I get an "isIncomplete(con)" error when knitting to PDF. This package requires use of at least 2 cores, and I'm wondering if this connection error is related to trying to run in parallel.
---
title: "Test"
output: pdf_document
---
```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```
```{r}
install.packages("http://www.statistica.it/gianluca/SWSamp/SWSamp_0.3.tar.gz", type="source", dependencies=TRUE, repos=NULL)
library(SWSamp)
```
```{r}
mu1=0.3
b.trt=-0.3875
sigma.e=1.55
J=5
K=20
sig.level=0.05
n.sims=10
rho=0.5
pow.cont <- sim.power(I=14, J=J, H=NULL, K=K, rho=rho, mu=mu1, sigma.e=sigma.e,
b.trt=b.trt, formula=NULL, n.sims=n.sims,
sig.level=sig.level, n.cores=2)
pow.cont$power
```
Quitting from lines 16-28 (test.Rmd) Error in isIncomplete(con) :
invalid connection Calls: ... evaluate_call ->
handle_output -> -> isIncomplete Quitting from lines 16-28
(test.Rmd) Error in isOpen(con) : invalid connection Calls:
... in_dir -> evaluate -> evaluate_call -> ->
isOpen Error in close.connection(con) : invalid connection Calls:
-> -> close -> close.connection Execution
halted
> sessionInfo()
R version 3.3.2 (2016-10-31)
Platform: x86_64-apple-darwin13.4.0 (64-bit)
Running under: macOS Sierra 10.12.1
locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] SWSamp_0.3 lme4_1.1-12 Matrix_1.2-7.1
loaded via a namespace (and not attached):
[1] Rcpp_0.12.8 knitr_1.15.1 magrittr_1.5 splines_3.3.2 MASS_7.3-45
[6] doParallel_1.0.10 lattice_0.20-34 foreach_1.4.3 minqa_1.2.4 stringr_1.1.0
[11] tools_3.3.2 parallel_3.3.2 grid_3.3.2 nlme_3.1-128 htmltools_0.3.5
[16] iterators_1.0.8 yaml_2.1.14 rprojroot_1.1 digest_0.6.10 nloptr_1.0.4
[21] codetools_0.2-15 evaluate_0.10 rmarkdown_1.3 stringi_1.1.2 compiler_3.3.2
[26] backports_1.0.4

Answer 1

The sim.power() function runs closeAllConnections(), which interferes with knitr according to this SO answer. The solution in this answer was to wrap the expression in quotes and the evaluate() function, but it did not work for me.
As a temporary solution, I redefined the sim.power() function with the closeAllConnections() call commented out. I don't know if this has adverse effects, but seems to run for me. Happy to accept better answers.