diff --git a/core/session_api.php b/core/session_api.php
index ff1230a835..9f2ab4df7f 100644
--- a/core/session_api.php
+++ b/core/session_api.php
@@ -171,6 +171,7 @@ function destroy() {
 		}
 
 		unset( $_SESSION[ $this->key ] );
+		session_write_close();
 	}
 }
 
diff --git a/signup.php b/signup.php
index 37f3f2725f..af5324f033 100644
--- a/signup.php
+++ b/signup.php
@@ -37,6 +37,9 @@
 	$f_email = email_append_domain( trim( $f_email ) );
 	$f_captcha = utf8_strtolower( trim( $f_captcha ) );
 
+	# Retrieve captcha key now, as session might get cleared by logout
+	$t_form_key = session_get_int( CAPTCHA_KEY, null );
+
 	# force logout on the current user if already authenticated
 	if( auth_is_user_authenticated() ) {
 		auth_logout();
@@ -50,8 +53,6 @@
 
 	if( ON == config_get( 'signup_use_captcha' ) && get_gd_version() > 0 	&&
 				helper_call_custom_function( 'auth_can_change_password', array() ) ) {
-		$t_form_key = session_get( CAPTCHA_KEY );
-
 		# captcha image requires GD library and related option to ON
 		$t_key = utf8_strtolower( utf8_substr( md5( config_get( 'password_confirm_hash_magic_string' ) . $t_form_key ), 1, 5) );