diff --git a/src/gui/interact/ui-coord.hpp b/src/gui/interact/ui-coord.hpp
index d313ca876..daab564cb 100644
--- a/src/gui/interact/ui-coord.hpp
+++ b/src/gui/interact/ui-coord.hpp
@@ -290,6 +290,7 @@ namespace interact {
       
       /* === String representation === */
       
+      explicit
       operator string()  const
         {
           if (isnil (*this))