String interpolation

I am using Net::HTTP.get(uri) with the following uri
"http://mapquestapi.com/directions/v1/route?key&fro...

The problem is with the #{destination}


  addressFile = File.open("Alarm.txt")#File contains the text 100 Avery
St
  address = addressFile.read()
  puts address
  destination = address.gsub!(' ','+')
  addressFile.close()

When I try to interpolate the destination variable like this I get a
format
error. However this works..

  address = "100 Avery St"
  destination = address.gsub!(' ','+')

What am I missing?