diff --git a/src/V3Tristate.cpp b/src/V3Tristate.cpp
index ad30f8fd6..587aa572c 100644
--- a/src/V3Tristate.cpp
+++ b/src/V3Tristate.cpp
@@ -1155,7 +1155,9 @@ class TristateVisitor : public TristateBaseVisitor {
             AstVar* outModVarp = static_cast<AstVar*>(nodep->modVarp()->user4p());
             if (!outModVarp) {
                 // At top, no need for __out as might be input only. Otherwise resolvable.
-                UASSERT_OBJ(m_modp->isTop(), nodep, "Unlinked");
+                if (!m_modp->isTop()) {
+                    nodep->v3error("Unsupported: tristate in top-level IO: " << nodep->prettyNameQ());
+                }
             } else {
                 AstNode* outexprp = nodep->exprp()->cloneTree(false);  // Note has lvalue() set
                 outpinp = new AstPin(nodep->fileline(),
diff --git a/test_regress/t/t_tri_compass_bad.out b/test_regress/t/t_tri_compass_bad.out
new file mode 100644
index 000000000..ccfb73039
--- /dev/null
+++ b/test_regress/t/t_tri_compass_bad.out
@@ -0,0 +1,5 @@
+%Error: t/t_tri_compass_bad.v:15: Unsupported: tristate in top-level IO: '__pinNumber1'
+                                : ... In instance t
+   sub sub(i, o);
+           ^
+%Error: Exiting due to
diff --git a/test_regress/t/t_tri_compass_bad.pl b/test_regress/t/t_tri_compass_bad.pl
new file mode 100755
index 000000000..3264f888b
--- /dev/null
+++ b/test_regress/t/t_tri_compass_bad.pl
@@ -0,0 +1,18 @@
+#!/usr/bin/perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003 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.
+
+scenarios(vlt => 1);
+
+compile(
+    expect_filename => $Self->{golden_filename},
+    fails => 1,
+    );
+
+ok(1);
+1;
diff --git a/test_regress/t/t_tri_compass_bad.v b/test_regress/t/t_tri_compass_bad.v
new file mode 100644
index 000000000..abe36c903
--- /dev/null
+++ b/test_regress/t/t_tri_compass_bad.v
@@ -0,0 +1,20 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed into the Public Domain, for any use,
+// without warranty, 2020 by Wilson Snyder.
+
+module t(/*AUTOARG*/
+   // Outputs
+   o,
+   // Inputs
+   i
+   );
+
+   input i;
+   output o;
+   sub sub(i, o);
+endmodule
+
+module sub(input i, output o);
+   assign o = (i===1'bz) ? 1'b0 : i;
+endmodule