diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index 98573ecd3b612..fd6e809421838 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -113,6 +113,7 @@
 		"./vs/base/test/browser/ui/tree/indexTreeModel.test.ts",
 		"./vs/base/test/browser/ui/tree/objectTree.test.ts",
 		"./vs/base/test/browser/ui/tree/objectTreeModel.test.ts",
+		"./vs/base/test/common/arrays.test.ts",
 		"./vs/base/test/common/assert.test.ts",
 		"./vs/base/test/common/cache.test.ts",
 		"./vs/base/test/common/cancellation.test.ts",
diff --git a/src/vs/base/test/common/arrays.test.ts b/src/vs/base/test/common/arrays.test.ts
index dbc2162bc1d9b..38a235ea87163 100644
--- a/src/vs/base/test/common/arrays.test.ts
+++ b/src/vs/base/test/common/arrays.test.ts
@@ -262,7 +262,7 @@ suite('Arrays', () => {
 	}
 
 	test('coalesce', () => {
-		let a = arrays.coalesce([null, 1, null, 2, 3]);
+		let a: Array<number | undefined | null> = arrays.coalesce([null, 1, null, 2, 3]);
 		assert.equal(a.length, 3);
 		assert.equal(a[0], 1);
 		assert.equal(a[1], 2);
@@ -298,7 +298,7 @@ suite('Arrays', () => {
 	});
 
 	test('coalesce - inplace', function () {
-		let a = [null, 1, null, 2, 3];
+		let a: Array<number | undefined | null> = [null, 1, null, 2, 3];
 		arrays.coalesceInPlace(a);
 		assert.equal(a.length, 3);
 		assert.equal(a[0], 1);