diff --git a/src/expressions/functions/builtInFunctions_arrays.ts b/src/expressions/functions/builtInFunctions_arrays.ts
index fd122ebcd..f4f772d89 100644
--- a/src/expressions/functions/builtInFunctions_arrays.ts
+++ b/src/expressions/functions/builtInFunctions_arrays.ts
@@ -460,17 +460,16 @@ const arraySort: FunctionDefinitionType = (
 	});
 };
 
-function flattenItem(flatteneditems: ISequence, item: Value): ISequence {
+function flattenItem(item: Value): ISequence {
 	if (isSubtypeOf(item.type, ValueType.ARRAY)) {
-		return (item as ArrayValue).members.reduce(
-			(flatteneditemsOfMember, member) =>
-				member().mapAll((allValues) =>
-					allValues.reduce(flattenItem, flatteneditemsOfMember)
-				),
-			flatteneditems
+		const arrayItem = item as ArrayValue;
+		return concatSequences(
+			arrayItem.members.map((member) =>
+				member().mapAll((subItems) => concatSequences(subItems.map(flattenItem)))
+			)
 		);
 	}
-	return concatSequences([flatteneditems, sequenceFactory.singleton(item)]);
+	return sequenceFactory.singleton(item);
 }
 const arrayFlatten: FunctionDefinitionType = (
 	_dynamicContext,
@@ -478,7 +477,7 @@ const arrayFlatten: FunctionDefinitionType = (
 	_staticContext,
 	itemSequence
 ) => {
-	return itemSequence.mapAll((items) => items.reduce(flattenItem, sequenceFactory.empty()));
+	return itemSequence.mapAll((items) => concatSequences(items.map(flattenItem)));
 };
 
 const declarations: BuiltinDeclarationType[] = [
diff --git a/test/specs/parsing/arrays/arrayFunctions.tests.ts b/test/specs/parsing/arrays/arrayFunctions.tests.ts
index d98784308..cfef7b751 100644
--- a/test/specs/parsing/arrays/arrayFunctions.tests.ts
+++ b/test/specs/parsing/arrays/arrayFunctions.tests.ts
@@ -3,6 +3,7 @@ import {
 	evaluateXPath,
 	evaluateXPathToArray,
 	evaluateXPathToBoolean,
+	evaluateXPathToNumber,
 	evaluateXPathToNumbers,
 	evaluateXPathToString,
 	evaluateXPathToStrings,
@@ -433,6 +434,21 @@ describe('functions over arrays', () => {
 				evaluateXPathToStrings('array:flatten(["a", ["b", "c"], "d"])', documentNode),
 				['a', 'b', 'c', 'd']
 			));
+
+		it('can flatten long arrays', () =>
+			chai.assert.equal(
+				evaluateXPathToNumber('array:flatten(array{1 to 20000}) => count()', documentNode),
+				20000
+			));
+
+		it('can flatten deep arrays', () =>
+			chai.assert.equal(
+				evaluateXPathToNumber(
+					'(1 to 200) => fold-left([1], function ($accum, $item) {[$accum, $item]}) => array:flatten() => count()',
+					documentNode
+				),
+				201
+			));
 	});
 
 	describe('atomizing arrays', () => {