diff --git a/pcbnew/dialogs/dialog_design_rules.cpp b/pcbnew/dialogs/dialog_design_rules.cpp
index d63706e701..2a55429e50 100644
--- a/pcbnew/dialogs/dialog_design_rules.cpp
+++ b/pcbnew/dialogs/dialog_design_rules.cpp
@@ -249,6 +249,9 @@ void DIALOG_DESIGN_RULES::InitDialogRules()
     // Initialize the Rules List
     InitRulesList();
 
+    // Reassure that all nets have net classes assigned
+    m_Pcb->BuildListOfNets();
+
     // copy all NETs into m_AllNets by adding them as NETCUPs.
 
     // @todo go fix m_Pcb->SynchronizeNetsAndNetClasses() so that the netcode==0 is not