R API call fails due to certificate chain issued by an authority that is not trusted

I'm just starting to get my feet wet in the API waters, and I'm trying to follow along the "Send a simple request" section at this link. When executing

github_api <- function(path) {
  url <- modify_url("https://api.github.com", path = path)
  GET(url)
}

resp <- github_api("/repos/hadley/httr")

I get the following error message:

Error in curl::curl_fetch_memory(url, handle = handle) : schannel: next InitializeSecurityContext failed: SEC_E_UNTRUSTED_ROOT (0x80090325) - The certificate chain was issued by an authority that is not trusted.

I'm getting similar error messages for most API calls I try to make on my machine, although the call

GET("http://api.open-notify.org/astros.json")

taken from this link happily returns data without issue. Searching google for the error message returns a lot of posts unrelated to R specifically and I'm having trouble determining what troubleshooting steps I can take.

Update

I have tested the call on another machine with success, so there is some setting/configuration/firewall impediment on my main machine which is preventing me from making some, but not all, API calls. This may be related to this issue. Is there a way to determine the root cause here and apply a fix?

I was able to solve this by using a forward proxy which allows my machine to reach sites outside my corporate firewall as follows (I've obscured the url and port for obvious reasons):

proxy <- use_proxy( url = "http://myproxy"
                   ,port = 9999
                   ,auth = "basic")
github_api <- function(path) {
  url <- modify_url("https://api.github.com", path = path)
  GET(url, proxy)
}

resp <- github_api("/repos/hadley/httr")

Hopefully such a forward proxy exists for anyone else facing this issue.