diff --git a/source/simplify-deep.d.ts b/source/simplify-deep.d.ts
index 7fce736ce..ffbae5d49 100644
--- a/source/simplify-deep.d.ts
+++ b/source/simplify-deep.d.ts
@@ -1,5 +1,6 @@
 import type {ConditionalSimplifyDeep} from './conditional-simplify-deep.d.ts';
 import type {MapsSetsOrArrays, NonRecursiveType} from './internal/index.d.ts';
+import type {UnknownArray} from './unknown-array.d.ts';
 
 /**
 Deeply simplifies an object type.
@@ -109,7 +110,7 @@ type SimplifyDeepProperties = SimplifyDeep<Properties1 & Properties2, ComplexTyp
 export type SimplifyDeep<Type, ExcludeType = never> =
 	ConditionalSimplifyDeep<
 		Type,
-	ExcludeType | NonRecursiveType | MapsSetsOrArrays,
+	ExcludeType | NonRecursiveType | Exclude<MapsSetsOrArrays, UnknownArray>,
 	object
 	>;
 
diff --git a/test-d/simplify-deep.ts b/test-d/simplify-deep.ts
index 9b539e14d..a4edfa5c7 100644
--- a/test-d/simplify-deep.ts
+++ b/test-d/simplify-deep.ts
@@ -32,13 +32,14 @@ declare const flattenProperties: {
 expectType<SimplifyDeep<Properties1 & Properties2>>(flattenProperties);
 
 // Array
-type ArrayType = Array<{
-	a: string;
-}>;
+declare function testArraySimplification(arg: {foo: Array<{[x: string]: string}>}): void;
 
-declare const flattenProperties2: {
-	arrayType: Array<{
-		a: string;
-	}>;
-};
-expectType<SimplifyDeep<{arrayType: ArrayType}>>(flattenProperties2);
+// eslint-disable-next-line @typescript-eslint/consistent-type-definitions
+interface BarBaz {
+	bar: string;
+	baz: string;
+}
+
+// This would fail if `SimplifyDeep` did not simplify arrays,
+// because interfaces being open are not compatible with index signatures.
+testArraySimplification({} as SimplifyDeep<{foo: BarBaz[]}>);