diff --git a/src/compiler/transformers/declarations.ts b/src/compiler/transformers/declarations.ts
index 973b9db618e..b54cf5bb858 100644
--- a/src/compiler/transformers/declarations.ts
+++ b/src/compiler/transformers/declarations.ts
@@ -1100,7 +1100,8 @@ namespace ts {
                     if (extendsClause && !isEntityNameExpression(extendsClause.expression) && extendsClause.expression.kind !== SyntaxKind.NullKeyword) {
                         // We must add a temporary declaration for the extends clause expression
 
-                        const newId = createOptimisticUniqueName(`${unescapeLeadingUnderscores(input.name!.escapedText)}_base`); // TODO: GH#18217
+                        const oldId = input.name ? unescapeLeadingUnderscores(input.name.escapedText) : "default";
+                        const newId = createOptimisticUniqueName(`${oldId}_base`);
                         getSymbolAccessibilityDiagnostic = () => ({
                             diagnosticMessage: Diagnostics.extends_clause_of_exported_class_0_has_or_is_using_private_name_1,
                             errorNode: extendsClause,