diff --git a/index.go b/index.go
index 5f6e7190c..6108d514f 100644
--- a/index.go
+++ b/index.go
@@ -341,7 +341,9 @@ func (i *Index) openField(mu *sync.Mutex, cfm *CreateFieldMessage, file string)
 	}
 
 	i.holder.Logger.Debugf("add field to index.fields: %s", file)
+	mu.Lock()
 	i.fields[fld.Name()] = fld
+	mu.Unlock()
 
 	return fld, nil
 }