diff --git a/include/nsc.h b/include/nsc.h
index f46ca6cc..405cf46d 100644
--- a/include/nsc.h
+++ b/include/nsc.h
@@ -213,8 +213,10 @@ enum {
  * NP points to the country to use for coordinate translation and
  * access control (null for none), and CTXO is the context object.
  * See struct valstr for details.
+ * Because virtual selectors don't have a setter method, xundump must
+ * be made to ignore them, e.g. by setting NSC_EXTRA.
  * If flag NSC_DEITY is set, only to deities can use this selector.
- * If flag NSC_EXTRA is set, xdump ignores this selector.
+ * If flag NSC_EXTRA is set, xdump and xundump ignore this selector.
  * If flag NSC_CONST is set, the datum can't be changed from its
  * initial value (xundump obeys that).
  * If ca_table is not EF_BAD, the datum refers to that Empire table;
diff --git a/src/lib/common/xundump.c b/src/lib/common/xundump.c
index 1cce54f7..69914c33 100644
--- a/src/lib/common/xundump.c
+++ b/src/lib/common/xundump.c
@@ -47,6 +47,7 @@
  * - Check each partial table supplies the same rows
  * - Check EFF_CFG tables are dense
  * - Symbolic array indexes
+ * - Option to treat missing and unknown fields as warning, not error
  * TODO, but hardly worth the effort:
  * - Permit reordering of array elements
  */
@@ -352,7 +353,7 @@ deffld(int fldno, char *name, int idx)
     if (res < 0)
 	return gripe("Header %s of field %d is %s", name, fldno + 1,
 		     res == M_NOTUNIQUE ? "ambiguous" : "unknown");
-    if (ca[res].ca_flags == NSC_EXTRA)
+    if (ca[res].ca_flags == NSC_EXTRA || CANT_HAPPEN(ca[res].ca_get))
 	return gripe("Extraneous header %s in field %d", name, fldno + 1);
     if (ca[res].ca_type != NSC_STRINGY && ca[res].ca_len != 0) {
 	if (idx < 0)