diff --git a/ngcp/avp.lua b/ngcp/avp.lua
index 6bfaca9..04ebade 100644
--- a/ngcp/avp.lua
+++ b/ngcp/avp.lua
@@ -1,5 +1,3 @@
-#!/usr/bin/env lua5.1
-
 -- class NGCPAvp
 NGCPAvp = {
      __class__ = 'NGCPAvp'
diff --git a/ngcp/pref.lua b/ngcp/pref.lua
index a49c6f8..e6fbbca 100644
--- a/ngcp/pref.lua
+++ b/ngcp/pref.lua
@@ -1,4 +1,4 @@
-#!/usr/bin/env lua5.1
+require 'ngcp.avp'
 require 'ngcp.xavp'
 
 -- class NGCPPrefs
@@ -15,5 +15,22 @@ NGCPPrefs_MT = { __index = NGCPPrefs }
         end
     end
 
+    function NGCPPrefs.set_avp(avp_name, xavp_name, default)
+    	local xavp
+    	local avp = NGCPAvp:new(avp_name)
+
+    	if xavp_name then
+    		xavp = sr.pv.get("$xavp(" .. xavp_name .. ")")
+    	else
+    		avp:clean()
+    		return
+    	end
+
+    	if default and not xavp then
+    		avp(default)
+    	else
+    		avp(xavp)
+    	end
+    end
 -- class
 --EOF
\ No newline at end of file
diff --git a/tests/ngcp_pref.lua b/tests/ngcp_pref.lua
new file mode 100644
index 0000000..8b2ac56
--- /dev/null
+++ b/tests/ngcp_pref.lua
@@ -0,0 +1,26 @@
+require('luaunit')
+require 'ngcp.pref'
+
+TestNGCPPrefs = {} --class
+
+	function TestNGCPPrefs:tearDown()
+		sr.pv.vars = {}
+	end
+
+    function TestNGCPPrefs:test_set_avp_empty()
+        sr.pv.sets("$avp(s:loquesea)", "one")
+        assertEquals(sr.pv.get("$avp(s:loquesea)"),"one")
+        NGCPPrefs.set_avp("loquesea")
+        assertEquals(sr.pv.get("$avp(s:loquesea)"), nil)
+    end
+
+    function TestNGCPPrefs:test_set_avp_val()
+        sr.pv.sets("$avp(s:loquesea)", "one")
+        sr.pv.sets("$xavp(callee_peer_prefs=>hola)", "two")
+        assertEquals(sr.pv.get("$avp(s:loquesea)"),"one")
+        assertEquals(sr.pv.get("$xavp(callee_peer_prefs[0]=>hola)"),"two")
+        NGCPPrefs.set_avp("loquesea", "callee_peer_prefs=>hola")
+        assertEquals(sr.pv.get("$avp(s:loquesea)"), "two")
+    end
+-- class TestNGCP
+--EOF
\ No newline at end of file
diff --git a/tests/test_all.lua b/tests/test_all.lua
index e581696..b43bdb7 100644
--- a/tests/test_all.lua
+++ b/tests/test_all.lua
@@ -1,6 +1,8 @@
 require 'tests.utils'
 require 'tests.mocks'
+require 'tests.ngcp_avp'
 require 'tests.ngcp_xavp'
+require 'tests.ngcp_pref'
 require 'tests.ngcp_dp'
 require 'tests.ngcp_up'
 require 'tests.ngcp_pp'
diff --git a/tests/test_kamailio.lua b/tests/test_kamailio.lua
index 266fa34..017c5a7 100644
--- a/tests/test_kamailio.lua
+++ b/tests/test_kamailio.lua
@@ -1,5 +1,7 @@
 require 'tests.utils'
+require 'tests.ngcp_avp'
 require 'tests.ngcp_xavp'
+require 'tests.ngcp_pref'
 require 'tests.ngcp_dp'
 require 'tests.ngcp_up'
 require 'tests.ngcp_pp'