diff --git a/app/switch/resources/scripts/app/fax/resources/scripts/hangup_rx.lua b/app/switch/resources/scripts/app/fax/resources/scripts/hangup_rx.lua
index d4cce11e28..0c17327488 100644
--- a/app/switch/resources/scripts/app/fax/resources/scripts/hangup_rx.lua
+++ b/app/switch/resources/scripts/app/fax/resources/scripts/hangup_rx.lua
@@ -63,6 +63,7 @@
 
 --escape shell arguments to prevent command injection
 	local function shell_esc(x)
+		if (x == nil) then return '' end
 		return (x:gsub('\\', '\\\\')
 			:gsub('\'', '\\\''))
 	end