diff --git a/testcases/parser/parse_test.go b/testcases/parser/parse_test.go
index 7e77056..0b67a20 100644
--- a/testcases/parser/parse_test.go
+++ b/testcases/parser/parse_test.go
@@ -101,6 +101,42 @@ lt('2016-12-31T13:30:15'::ts, '2017-12-31T13:30:15'::ts) = true::bool
 	assert.Equal(t, ScalarFuncType, testFile.TestCases[0].FuncType)
 }
 
+// Regression for #237: a `null::u!<identifier>[?]` argument used to panic
+// in `VisitNullArg` because `TestCaseVisitor` had no `VisitUserDefined`
+// method, so dispatch fell through to the base visitor's `VisitChildren`
+// (which returns nil for the terminal `UserDefinedContext` node). The new
+// `VisitUserDefined` returns a `*types.UserDefinedType` so the null
+// literal can be constructed without panic. (`VisitNullArg` then forces
+// the `CaseLiteral.Type`'s nullability to `Nullable` regardless of the
+// parsed `?`, so we don't assert on that — we only assert the arg type
+// is a `UserDefinedType` and the value is a `NullLiteral`.)
+func TestParseUserDefinedNullArg(t *testing.T) {
+	header := makeHeader("v1.0", "/extensions/functions_arithmetic.yaml")
+	groupDesc := "# user-defined null arg parsing (issue #237)\n"
+	cases := []string{
+		"f1(null::u!geometry) = 1::i8",
+		"f1(null::u!geometry?) = 1::i8",
+	}
+	for _, tc := range cases {
+		t.Run(tc, func(t *testing.T) {
+			testFile, err := ParseTestCasesFromString(header + groupDesc + tc)
+			require.NoError(t, err, "parse must not panic / error on user-defined null arg")
+			require.NotNil(t, testFile)
+			require.Len(t, testFile.TestCases, 1)
+			require.Len(t, testFile.TestCases[0].Args, 1)
+
+			arg := testFile.TestCases[0].Args[0]
+			_, ok := arg.Type.(*types.UserDefinedType)
+			require.True(t, ok, "expected *types.UserDefinedType, got %T", arg.Type)
+
+			lit, ok := arg.Value.(*expr.NullLiteral)
+			require.True(t, ok, "expected *expr.NullLiteral, got %T", arg.Value)
+			_, ok = lit.GetType().(*types.UserDefinedType)
+			assert.True(t, ok, "NullLiteral.GetType() should be *types.UserDefinedType, got %T", lit.GetType())
+		})
+	}
+}
+
 func TestParseDecimalExample(t *testing.T) {
 	header := makeHeader("v1.0", "extensions/functions_arithmetic_decimal.yaml")
 	tests := `# basic
diff --git a/testcases/parser/visitor.go b/testcases/parser/visitor.go
index 53e6bb1..623fe9a 100644
--- a/testcases/parser/visitor.go
+++ b/testcases/parser/visitor.go
@@ -944,6 +944,25 @@ func (v *TestCaseVisitor) VisitDataType(ctx *baseparser.DataTypeContext) interfa
 	return nil
 }
 
+// VisitUserDefined handles the `u!<identifier>[?]` scalar form that
+// shows up in parser inputs like `null::u!geometry?`. The base visitor
+// returns VisitChildren here (nil), which caused VisitNullArg to try
+// to use nil as a types.Type and panic with
+// "interface conversion: interface {} is nil, not string".
+func (v *TestCaseVisitor) VisitUserDefined(ctx *baseparser.UserDefinedContext) interface{} {
+	nullability := types.NullabilityRequired
+	if ctx.QMark() != nil {
+		nullability = types.NullabilityNullable
+	}
+	ident := ctx.Identifier()
+	if ident == nil {
+		v.ErrorListener.ReportVisitError(ctx, fmt.Errorf("user-defined type without identifier: %v", ctx.GetText()))
+		return &types.UserDefinedType{Nullability: nullability}
+	}
+	_ = ident.GetText() // reserved for type-lookup; not available at this visit depth
+	return &types.UserDefinedType{Nullability: nullability}
+}
+
 func (v *TestCaseVisitor) VisitParameterizedType(ctx *baseparser.ParameterizedTypeContext) interface{} {
 	if ctx.DecimalType() != nil {
 		return v.Visit(ctx.DecimalType())