diff --git a/modules/exploits/linux/pptp/poptop_negative_read.rb b/modules/exploits/linux/pptp/poptop_negative_read.rb
index dfe3e0095b..607b3e2a8d 100644
--- a/modules/exploits/linux/pptp/poptop_negative_read.rb
+++ b/modules/exploits/linux/pptp/poptop_negative_read.rb
@@ -73,7 +73,7 @@ class Exploits::Linux::Pptp::PoptopNegativeRead < Msf::Exploit::Remote
 
 	# Dynamic payload space calculation
 	def payload_space
-		datastore['PreReturnLength'] + datastore['ExtraSpace']
+		datastore['PreReturnLength'].to_i + datastore['ExtraSpace'].to_i
 	end
 
 	def build_packet(length)