diff --git a/models/workflow/workflow_mongo_accessor.go b/models/workflow/workflow_mongo_accessor.go
index bf32738..215bdfc 100644
--- a/models/workflow/workflow_mongo_accessor.go
+++ b/models/workflow/workflow_mongo_accessor.go
@@ -82,6 +82,9 @@ func (wfa *workflowMongoAccessor) DeleteOne(id string) (utils.DBObject, int, err
 }
 
 func (wfa *workflowMongoAccessor) book(id string, realData *Workflow, execs []*workflow_execution.WorkflowExecution) error {
+	if wfa.Caller == nil {
+		return errors.New("no caller defined")
+	}
 	if realData.Schedule == nil {
 		return nil
 	}