What causes "Neither PUB key nor PRIV key:: nested asn1 error" when building a public key in ruby?

When building a public key using the OpenSSL::PKey::RSA module by passing it a .pem file, what is the cause for a response:

OpenSSL::PKey::RSAError: Neither PUB key nor PRIV key:: nested asn1 error
from /Users/Matt/projects/placepop/lib/apn.rb:48:in `initialize'
from /Users/Matt/projects/placepop/lib/apn.rb:48:in `new'
from /Users/Matt/projects/placepop/lib/apn.rb:48:in `open'
from (irb):1

Here is the source:

cert = File.join(rails_root, 'config', 'apns', 'sandbox-cert.pem')
APN_CONFIG = { :delivery => { 
                              :host => 'gateway.sandbox.push.apple.com', 
                              :cert => cert,
                              :passphrase => "",
                              :port => 2195 },
               :feedback => {  
                              :host => 'feedback.sandbox.push.apple.com',
                              :port => 2196,
                              :passphrase => "",
                              :cert => cert} }


options = APN_CONFIG[:delivery].merge(options)
cert = File.read(options[:cert])
ctx = OpenSSL::SSL::SSLContext.new
ctx.key = OpenSSL::PKey::RSA.new(cert, options[:passphrase])
ctx.cert = OpenSSL::X509::Certificate.new(cert)

sock = TCPSocket.new(options[:host], options[:port])
ssl = OpenSSL::SSL::SSLSocket.new(sock, ctx)
ssl.sync = true
ssl.connect

A pem file is not a public key, it is a base64-encoded X509 certificate that contains, among its many fields, a public key. I don't know Ruby, or the OpenSSL ruby module, but I would look for some function that reads in PEM files and outputs an X509 certificate, then another function to extract the public key from the certificate.