diff --git a/CHANGES b/CHANGES
index 09a4107696..28c7834366 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,7 @@
+8.1.0-dev.631 | 2025-10-03 14:07:06 -0700
+
+  * fix to enable -O gen-standalone-C++ code to extend enum types (Vern Paxson, Corelight)
+
 8.1.0-dev.629 | 2025-10-03 09:13:28 -0700
 
   * Fix some recent Coverity findings (Tim Wojtulewicz, Corelight)
diff --git a/VERSION b/VERSION
index 57f82aabef..cefb9477a8 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-8.1.0-dev.629
+8.1.0-dev.631
diff --git a/src/Type.cc b/src/Type.cc
index 397a27ebe8..d461f3c1e5 100644
--- a/src/Type.cc
+++ b/src/Type.cc
@@ -1700,8 +1700,11 @@ void EnumType::AddNameInternal(const string& full_name, zeek_int_t val) {
     names[full_name] = val;
     rev_names[val] = full_name;
 
-    if ( ! vals.contains(val) )
+    if ( ! vals.contains(val) ) {
+        if ( counter >= 0 && val == static_cast<zeek_int_t>(vals.size()) )
+            ++counter;
         vals[val] = make_intrusive<EnumVal>(IntrusivePtr{NewRef{}, this}, val);
+    }
 }
 
 zeek_int_t EnumType::Lookup(const string& module_name, const char* name) const {