diff --git a/htdocs/product/dynamic_price/class/price_global_variable_updater.class.php b/htdocs/product/dynamic_price/class/price_global_variable_updater.class.php
index 42030aa290e..c0ec8226e3d 100644
--- a/htdocs/product/dynamic_price/class/price_global_variable_updater.class.php
+++ b/htdocs/product/dynamic_price/class/price_global_variable_updater.class.php
@@ -456,7 +456,7 @@ class PriceGlobalVariableUpdater
 		$langs->load("errors");
 		dol_syslog(__METHOD__, LOG_DEBUG);
 
-		$this->error = null;
+		$this->error = '';
 		$this->checkParameters();
 
 		//Try to load the target global variable and abort if fails