diff --git a/absl/flags/parse.cc b/absl/flags/parse.cc
index 4f4bb3d..1835a83 100644
--- a/absl/flags/parse.cc
+++ b/absl/flags/parse.cc
@@ -611,6 +611,11 @@
                                         OnUndefinedFlag on_undef_flag) {
   ABSL_INTERNAL_CHECK(argc > 0, "Missing argv[0]");
 
+  // Once parsing has started we will not have more flag registrations.
+  // If we did, they would be missing during parsing, which is a problem on
+  // itself.
+  flags_internal::FinalizeRegistry();
+
   // This routine does not return anything since we abort on failure.
   CheckDefaultValuesParsingRoundtrip();