diff --git a/lib/net/snmp/session.rb b/lib/net/snmp/session.rb
index 1536038..92e16c1 100644
--- a/lib/net/snmp/session.rb
+++ b/lib/net/snmp/session.rb
@@ -136,7 +136,7 @@ def initialize(options = {})
             auth_key_result = Wrapper.generate_Ku(@sess.securityAuthProto,
                                              @sess.securityAuthProtoLen,
                                              options[:auth_password],
-                                             options[:auth_password].length,
+                                             options[:auth_password].bytesize,
                                              @sess.securityAuthKey,
                                              auth_len_ptr)
             @sess.securityAuthKeyLen = auth_len_ptr.read_int
@@ -150,7 +150,7 @@ def initialize(options = {})
               priv_key_result = Wrapper.generate_Ku(@sess.securityAuthProto,
                                                @sess.securityAuthProtoLen,
                                                options[:priv_password],
-                                               options[:priv_password].length,
+                                               options[:priv_password].bytesize,
                                                @sess.securityPrivKey,
                                                priv_len_ptr)
               @sess.securityPrivKeyLen = priv_len_ptr.read_int
@@ -171,10 +171,12 @@ def initialize(options = {})
       def close
         if Net::SNMP.thread_safe
           self.class.lock.synchronize {
-            Wrapper.snmp_sess_close(@struct)
+            Wrapper..shutdown_usm()
+	    Wrapper.snmp_sess_close(@struct)
             self.class.sessions.delete(self.sessid)
           }
         else
+          Wrapper.shutdown_usm()
           Wrapper.snmp_sess_close(@struct)
           self.class.sessions.delete(self.sessid)
         end
diff --git a/lib/net/snmp/wrapper.rb b/lib/net/snmp/wrapper.rb
index 0a81cb2..4b717a9 100644
--- a/lib/net/snmp/wrapper.rb
+++ b/lib/net/snmp/wrapper.rb
@@ -358,6 +358,8 @@ class IndexList < NiceFFI::Struct
   #attach_function :send_easy_trap, [:int, :int], :void
   #attach_function :send_trap_vars, [:int, :int, :pointer], :void
   #attach_function :send_v2trap, [:pointer], :void
+  
+  attach_function :shutdown_usm, [], :void
 
   def self.get_fd_set
     FFI::MemoryPointer.new(:pointer, 128)