diff --git a/__tests__/type-external.ts b/__tests__/type-external.ts
index fcf3b2df..496a5557 100644
--- a/__tests__/type-external.ts
+++ b/__tests__/type-external.ts
@@ -1,6 +1,22 @@
 import {isType, JSONArray, JSONObject, JSONTypes} from "type-plus"
 import {Draft} from "../src/types/types-external"
 import {createDraft, current, original} from "../src/immer"
+import ts from "typescript"
+
+test("source compiles with noUncheckedIndexedAccess (#1143)", () => {
+	const {config, error} = ts.readConfigFile("tsconfig.json", ts.sys.readFile)
+	expect(error).toBeUndefined()
+	const parsed = ts.parseJsonConfigFileContent(config, ts.sys, ".", {
+		noUncheckedIndexedAccess: true,
+		skipLibCheck: true
+	})
+	const program = ts.createProgram(parsed.fileNames, parsed.options)
+	expect(
+		[...parsed.errors, ...ts.getPreEmitDiagnostics(program)].map(diagnostic =>
+			ts.flattenDiagnosticMessageText(diagnostic.messageText, "\n")
+		)
+	).toEqual([])
+})
 
 describe("Draft<T>", () => {
 	test("can use JSONTypes as T", () => {
diff --git a/src/core/immerClass.ts b/src/core/immerClass.ts
index 2922f612..224604b1 100644
--- a/src/core/immerClass.ts
+++ b/src/core/immerClass.ts
@@ -207,7 +207,7 @@ export class Immer implements ProducersFns {
 		// before applying patches
 		let i: number
 		for (i = patches.length - 1; i >= 0; i--) {
-			const patch = patches[i]
+			const patch = patches[i]!
 			if (patch.path.length === 0 && patch.op === "replace") {
 				base = patch.value
 				break
diff --git a/src/plugins/patches.ts b/src/plugins/patches.ts
index 08c7d0f6..b3c86f86 100644
--- a/src/plugins/patches.ts
+++ b/src/plugins/patches.ts
@@ -116,7 +116,7 @@ export function enablePatches() {
 	function resolvePath(base: any, path: PatchPath): any {
 		let current = base
 		for (let i = 0; i < path.length - 1; i++) {
-			const key = path[i]
+			const key = path[i]!
 			current = get(current, key)
 			if (!isObjectish(current) || current === null) {
 				throw new Error(`Cannot resolve path at '${path.join("/")}'`)
@@ -351,7 +351,7 @@ export function enablePatches() {
 
 			const type = getArchtype(base)
 			const value = deepClonePatchValue(patch.value) // used to clone patch to ensure original patch is not modified, see #411
-			const key = path[path.length - 1]
+			const key = path[path.length - 1]!
 			switch (op) {
 				case REPLACE:
 					switch (type) {
diff --git a/src/utils/common.ts b/src/utils/common.ts
index 513c153e..43f35ec0 100644
--- a/src/utils/common.ts
+++ b/src/utils/common.ts
@@ -215,7 +215,7 @@ export function shallowCopy(base: any, strict: StrictMode) {
 		let keys = Reflect.ownKeys(descriptors)
 		for (let i = 0; i < keys.length; i++) {
 			const key: any = keys[i]
-			const desc = descriptors[key]
+			const desc = descriptors[key]!
 			if (desc[WRITABLE] === false) {
 				desc[WRITABLE] = true
 				desc[CONFIGURABLE] = true