diff --git a/tests/cases/compiler/conditionalTypesASI.ts b/tests/cases/compiler/conditionalTypesASI.ts
new file mode 100644
index 00000000000..44d6d816851
--- /dev/null
+++ b/tests/cases/compiler/conditionalTypesASI.ts
@@ -0,0 +1,8 @@
+// @declaration: true
+
+// Repro from #21637
+
+interface JSONSchema4 {
+  a?: number
+  extends?: string | string[]
+}