diff --git a/src/V3Param.cpp b/src/V3Param.cpp
index 9ecfb4782..228ad6b8b 100644
--- a/src/V3Param.cpp
+++ b/src/V3Param.cpp
@@ -1199,20 +1199,20 @@ public:
 
 //######################################################################
 // This visitor records classes that are referenced with parameter pins
-class ClassPinsMarkVisitor final : public VNVisitorConst {
+class ClassRefUnlinkerVisitor final : public VNVisitor {
 
 public:
-    explicit ClassPinsMarkVisitor(AstNetlist* netlistp) { iterateConst(netlistp); }
+    explicit ClassRefUnlinkerVisitor(AstNetlist* netlistp) { iterate(netlistp); }
 
     void visit(AstClassOrPackageRef* nodep) override {
         if (nodep->paramsp()) {
             if (AstClass* const classp = VN_CAST(nodep->classOrPackageSkipp(), Class)) {
-                classp->user3p(classp);
+                if (!classp->user3p()) VL_DO_DANGLING(pushDeletep(nodep->unlinkFrBack()), nodep);
             }
         }
     }
     void visit(AstClass* nodep) override {}  // don't iterate inside classes
-    void visit(AstNode* nodep) override { iterateChildrenConst(nodep); }
+    void visit(AstNode* nodep) override { iterateChildren(nodep); }
 };
 
 //######################################################################
@@ -1683,7 +1683,7 @@ public:
         iterate(netlistp);
 
         // Mark classes which cannot be removed because they are still referenced
-        ClassPinsMarkVisitor markVisitor{netlistp};
+        ClassRefUnlinkerVisitor markVisitor{netlistp};
 
         relinkDots();
 
diff --git a/test_regress/t/t_bug6421.py b/test_regress/t/t_bug6421.py
new file mode 100755
index 000000000..6812c4542
--- /dev/null
+++ b/test_regress/t/t_bug6421.py
@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2025 by Wilson Snyder. This program is free software; you
+# can redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('vlt')
+
+test.compile(verilator_flags=["--binary"])
+
+test.passes()
diff --git a/test_regress/t/t_bug6421.v b/test_regress/t/t_bug6421.v
new file mode 100644
index 000000000..67e747b4c
--- /dev/null
+++ b/test_regress/t/t_bug6421.v
@@ -0,0 +1,38 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Antmicro.
+// SPDX-License-Identifier: CC0-1.0
+
+package uvm_pkg;
+class uvm_queue #(type T=int);
+endclass
+class m_uvm_waiter;
+endclass
+class uvm_config_db#(type T=int);
+  static local uvm_queue#(m_uvm_waiter) m_waiters[string];
+  static function void set(int a, string b, string c, int d);
+  endfunction
+endclass
+endpackage
+package sfr_agent_pkg;
+class sfr_monitor_abstract;
+endclass
+endpackage: sfr_agent_pkg
+module sfr_monitor_bfm  #(ADDR_WIDTH = 8,
+                          DATA_WIDTH = 8)
+                        (
+                         input [ADDR_WIDTH-1:0] address);
+  import uvm_pkg::*;
+  import sfr_agent_pkg::*; int SFR_MONITOR;
+initial begin
+  uvm_config_db #(sfr_monitor_abstract)::set(null, "uvm_test_top", "SFR_MONITOR", SFR_MONITOR);
+end
+endmodule: sfr_monitor_bfm
+module hdl_top;
+parameter DATA_WIDTH = 32;
+parameter ADDR_WIDTH = 32;
+sfr_monitor_bfm #(.ADDR_WIDTH(ADDR_WIDTH),
+                  .DATA_WIDTH(DATA_WIDTH)) SFR_MONITOR(
+                                                        .address(42));
+endmodule