diff --git a/src/index.ts b/src/index.ts
index d34a8f8..a403eff 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -65,7 +65,10 @@ export interface Instruction {
 	optionals: string[]
 	optionalsInArray: string[][]
 	parentIsOptional: boolean
-	array: number
+	/** Which array context we're currently inside (for storing optionals) */
+	currentArrayIndex: number
+	/** Mutable counter for allocating unique array indices to prevent sibling collisions */
+	nextArrayIndex: { value: number }
 	unions: TypeCheck<any>[][]
 	unionKeys: Record<string, 1>
 	sanitize: MaybeArray<(v: string) => string> | undefined
@@ -108,8 +111,8 @@ const handleRecord = (
 
 	if (!child) return property
 
-	const i = instruction.array
-	instruction.array++
+	const i = instruction.nextArrayIndex.value
+	instruction.nextArrayIndex.value++
 
 	let v =
 		`(()=>{` +
@@ -117,7 +120,7 @@ const handleRecord = (
 		`ar${i}v={};` +
 		`for(let i=0;i<ar${i}s.length;i++){` +
 		`const ar${i}p=${property}[ar${i}s[i]];` +
-		`ar${i}v[ar${i}s[i]]=${mirror(child, `ar${i}p`, instruction)}`
+		`ar${i}v[ar${i}s[i]]=${mirror(child, `ar${i}p`, { ...instruction, currentArrayIndex: i })}`
 
 	const optionals = instruction.optionalsInArray[i + 1]
 	if (optionals)
@@ -137,8 +140,8 @@ const handleTuple = (
 	property: string,
 	instruction: Instruction
 ) => {
-	const i = instruction.array
-	instruction.array++
+	const i = instruction.nextArrayIndex.value
+	instruction.nextArrayIndex.value++
 
 	const isRoot = property === 'v' && !instruction.unions.length
 
@@ -147,13 +150,13 @@ const handleTuple = (
 
 	v += `const ar${i}v=[`
 
-	for (let i = 0; i < schema.length; i++) {
-		if (i !== 0) v += ','
+	for (let idx = 0; idx < schema.length; idx++) {
+		if (idx !== 0) v += ','
 
 		v += mirror(
-			schema[i],
-			joinProperty(property, i, instruction.parentIsOptional),
-			instruction
+			schema[idx],
+			joinProperty(property, idx, instruction.parentIsOptional),
+			{ ...instruction, currentArrayIndex: i }
 		)
 	}
 
@@ -369,7 +372,8 @@ const mirror = (
 				)
 
 				if (isOptional) {
-					const index = instruction.array
+					// +1 because cleanup code uses optionalsInArray[i + 1] where i is captured BEFORE increment
+					const index = instruction.currentArrayIndex + 1
 
 					if (property.startsWith('ar')) {
 						const dotIndex = name.indexOf('.')
@@ -451,8 +455,8 @@ const mirror = (
 				}
 			}
 
-			const i = instruction.array
-			instruction.array++
+			const i = instruction.nextArrayIndex.value
+			instruction.nextArrayIndex.value++
 
 			let reference = property
 
@@ -467,7 +471,7 @@ const mirror = (
 			v +=
 				`for(let i=0;i<${reference}.length;i++){` +
 				`const ar${i}p=${reference}[i];` +
-				`ar${i}v[i]=${mirror(schema.items, `ar${i}p`, instruction)}`
+				`ar${i}v[i]=${mirror(schema.items, `ar${i}p`, { ...instruction, currentArrayIndex: i })}`
 
 			const optionals = instruction.optionalsInArray[i + 1]
 			if (optionals) {
@@ -562,7 +566,8 @@ export const createMirror = <T extends TAnySchema>(
 	const f = mirror(schema, 'v', {
 		optionals: [],
 		optionalsInArray: [],
-		array: 0,
+		currentArrayIndex: 0,
+		nextArrayIndex: { value: 0 },
 		parentIsOptional: false,
 		unions,
 		unionKeys: {},
diff --git a/test/sibling-arrays.test.ts b/test/sibling-arrays.test.ts
new file mode 100644
index 0000000..56faaed
--- /dev/null
+++ b/test/sibling-arrays.test.ts
@@ -0,0 +1,128 @@
+import { t } from 'elysia'
+
+import { describe, it } from 'bun:test'
+import { isEqual } from './utils'
+
+describe('Sibling Arrays', () => {
+	/**
+	 * Regression test for sibling array index collision bug.
+	 * Previously, sibling arrays would share the same optionalsInArray index,
+	 * causing cleanup code for one array to reference properties from another.
+	 */
+	it('handle sibling arrays with optionals in different objects', () => {
+		const shape = t.Object({
+			a: t.Array(
+				t.Object({
+					obj: t.Object({
+						n: t.Nullable(t.Object({ v: t.String() }))
+					})
+				})
+			),
+			b: t.Object({
+				arr: t.Array(t.Object({ x: t.Integer() }))
+			})
+		})
+
+		const value = {
+			a: [{ obj: { n: null } }],
+			b: { arr: [{ x: 1 }] }
+		}
+
+		// Should not throw "Cannot read properties of undefined (reading 'n')"
+		isEqual(shape, value)
+	})
+
+	it('handle sibling arrays at same depth with optionals', () => {
+		const shape = t.Object({
+			first: t.Array(
+				t.Object({
+					name: t.String(),
+					optional: t.Optional(t.String())
+				})
+			),
+			second: t.Array(
+				t.Object({
+					id: t.Number(),
+					optional: t.Optional(t.Number())
+				})
+			)
+		})
+
+		const value = {
+			first: [
+				{ name: 'a', optional: 'x' },
+				{ name: 'b' } // no optional
+			],
+			second: [
+				{ id: 1 },
+				{ id: 2, optional: 42 }
+			]
+		}
+
+		const expected = {
+			first: [
+				{ name: 'a', optional: 'x' },
+				{ name: 'b' }
+			],
+			second: [
+				{ id: 1 },
+				{ id: 2, optional: 42 }
+			]
+		}
+
+		isEqual(shape, value, expected)
+	})
+
+	it('handle nested arrays with optionals at multiple levels', () => {
+		const shape = t.Array(
+			t.Object({
+				name: t.String(),
+				games: t.Array(
+					t.Object({
+						id: t.Number(),
+						hoursPlay: t.Optional(t.Number())
+					})
+				),
+				// This optional should not be affected by games array processing
+				social: t.Optional(
+					t.Object({
+						twitter: t.Optional(t.String())
+					})
+				)
+			})
+		)
+
+		const value = [
+			{
+				name: 'user1',
+				games: [
+					{ id: 1, hoursPlay: 10 },
+					{ id: 2 }
+				],
+				social: { twitter: 'user1' }
+			},
+			{
+				name: 'user2',
+				games: [{ id: 3 }]
+				// no social
+			}
+		]
+
+		const expected = [
+			{
+				name: 'user1',
+				games: [
+					{ id: 1, hoursPlay: 10 },
+					{ id: 2 }
+				],
+				social: { twitter: 'user1' }
+			},
+			{
+				name: 'user2',
+				games: [{ id: 3 }]
+			}
+		]
+
+		isEqual(shape, value, expected)
+	})
+})