diff --git a/otp.lua b/otp.lua
index e5c3719..fb75e50 100644
--- a/otp.lua
+++ b/otp.lua
@@ -118,7 +118,7 @@ end
 
 function TOTP_MT:calc_token()
     local ngx_time = math.floor(ngx.time() / 30)
-    HMAC_Result_Final = {ngx.hmac_sha1(self.key_decoded, totp_time_calc(ngx_time)):byte(1, -1)}
+    local HMAC_Result_Final = {ngx.hmac_sha1(self.key_decoded, totp_time_calc(ngx_time)):byte(1, -1)}
 
     local HMAC_Offset = band(HMAC_Result_Final[20], 0xF)
 
@@ -172,4 +172,4 @@ function TOTP_MT:serialize()
     }
 end
 
-return _M
\ No newline at end of file
+return _M