diff --git a/lib/system/assign.nim b/lib/system/assign.nim
index e5ca1637c9..3bb52be4aa 100644
--- a/lib/system/assign.nim
+++ b/lib/system/assign.nim
@@ -237,4 +237,7 @@ proc FieldDiscriminantCheck(oldDiscVal, newDiscVal: int,
       sysFatal(FieldError, "assignment to discriminant changes object branch")
   else:
     if newBranch != oldBranch:
-      sysFatal(FieldError, "assignment to discriminant changes object branch")
+      if oldDiscVal != 0:
+        sysFatal(FieldError, "assignment to discriminant changes object branch")
+      else:
+        sysFatal(FieldError, "assignment to discriminant changes object branch; compile with -d:nimOldCaseObjects for a transition period")