diff --git a/compiler2/ttcn3/Statement.cc b/compiler2/ttcn3/Statement.cc
index c7692a69f19b23c431e0083e2ca7492982a05ef9..38e9a8955bb581ad342d3730b16fbd8e77afe2c9 100644
--- a/compiler2/ttcn3/Statement.cc
+++ b/compiler2/ttcn3/Statement.cc
@@ -4922,6 +4922,14 @@ error:
         config_op.portref2->warning("Port type `%s' was marked as `internal'",
                 pt2->get_typename().c_str());
       }
+      if ((ptb1 != NULL && !ptb1->is_legacy() &&
+           ptb1->get_type() == PortTypeBody::PT_USER) ||
+          (ptb2 != NULL && !ptb2->is_legacy() &&
+           ptb2->get_type() == PortTypeBody::PT_USER)) {
+        note("This mapping is not done in translation mode, because the %s "
+          "endpoint is unknown",
+          ptb1 != NULL ? "second" : "first");
+      }
       return;
     }
     if (cref1_is_tc || cref2_is_system) {
@@ -4970,6 +4978,10 @@ error:
         config_op.portref2->warning("Port type `%s' was marked as `internal'",
                 pt2->get_typename().c_str());
       }
+      if ((!ptb1->is_legacy() && ptb1->get_type() == PortTypeBody::PT_USER) ||
+          (!ptb2->is_legacy() && ptb2->get_type() == PortTypeBody::PT_USER)) {
+        note("This mapping is not done in translation mode");
+      }
     }
   }