Trouble using the r package (random)

I want to generate random string and have installed the random package. However, I couldn't run my code and received this error:

Error in url(urltxt, ..., method = "libcurl") : cannot open connection
In addition: Warning message: In url(urltxt, ..., method = "libcurl")
:   URL 'http://ift.tt/2lmRDkk'
:   status was 'SSL connect error'

Here's my code:

library("random")
String<-randomStrings(n=7, len = 6, digits = TRUE, upperalpha = TRUE, loweralpha = FALSE, unique = TRUE, check = FALSE)

Anybody knows what's the source of this error?