diff --git a/src/interfaces/node_arithmetic_base.php b/src/interfaces/node_arithmetic_base.php
index 4d41291..eda475a 100644
--- a/src/interfaces/node_arithmetic_base.php
+++ b/src/interfaces/node_arithmetic_base.php
@@ -9,9 +9,9 @@
  * to you under the Apache License, Version 2.0 (the
  * "License"); you may not use this file except in compliance
  * with the License.  You may obtain a copy of the License at
- * 
+ *
  *   http://www.apache.org/licenses/LICENSE-2.0
- * 
+ *
  * Unless required by applicable law or agreed to in writing,
  * software distributed under the License is distributed on an
  * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
@@ -111,7 +111,11 @@ public function execute( ezcWorkflowExecution $execution )
             );
         }
 
-        if ( is_numeric( $this->configuration['operand'] ) )
+        if ( !isset( $this->configuration['operand'] ) ) {
+            $this->operand = 1;
+        }
+
+        else if ( is_numeric( $this->configuration['operand'] ) )
         {
             $this->operand = $this->configuration['operand'];
         }