diff --git a/tests/cases/fourslash/completionsIndexSignatureConstraint1.ts b/tests/cases/fourslash/completionsIndexSignatureConstraint1.ts
new file mode 100644
index 0000000000000..7e67ee8db828a
--- /dev/null
+++ b/tests/cases/fourslash/completionsIndexSignatureConstraint1.ts
@@ -0,0 +1,29 @@
+/// <reference path="fourslash.ts" />
+// @strict: true
+////
+//// repro #9900
+////
+//// interface Test {
+////   a?: number;
+////   b?: string;
+//// }
+////
+//// interface TestIndex {
+////   [key: string]: Test;
+//// }
+////
+//// declare function testFunc<T extends TestIndex>(t: T): void;
+////
+//// testFunc({
+////   test: {
+////     /**/
+////   },
+//// });
+
+verify.completions({
+  marker: '',
+  exact: [
+    { name: 'a', sortText: completion.SortText.OptionalMember },
+    { name: 'b', sortText: completion.SortText.OptionalMember },
+  ]
+});
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