diff --git a/Iterator.concat/implementation.js b/Iterator.concat/implementation.js
index ca75c0f..435413c 100644
--- a/Iterator.concat/implementation.js
+++ b/Iterator.concat/implementation.js
@@ -11,16 +11,11 @@ var GetMethod = require('es-abstract/2024/GetMethod');
 var IsArray = require('es-abstract/2024/IsArray');
 var IteratorCloseAll = require('../aos/IteratorCloseAll');
 var IteratorStepValue = require('es-abstract/2024/IteratorStepValue');
-var ThrowCompletion = require('es-abstract/2024/ThrowCompletion');
 var Type = require('es-abstract/2024/Type');
 
 var forEach = require('es-abstract/helpers/forEach');
 var getIteratorMethod = require('es-abstract/helpers/getIteratorMethod');
 
-var callBound = require('call-bind/callBound');
-var $indexOf = callBound('Array.prototype.indexOf');
-var $splice = callBound('Array.prototype.splice');
-
 var iterHelperProto = require('../IteratorHelperPrototype');
 
 var SLOT = require('internal-slot');
@@ -51,65 +46,52 @@ module.exports = function concat() {
 		iterables[iterables.length] = { '[[OpenMethod]]': method, '[[Iterable]]': item }; // step 2.4
 	});
 
-	var openIters = []; // step 3
 	var sentinel = {};
+	var iterablesIndex = 0;
+	var iteratorRecord;
+	var innerAlive = false;
+	var closure = function () { // step 3
+		if (iterablesIndex >= iterables.length) {
+			return sentinel;
+		}
+		var iterable = iterables[iterablesIndex]; // step 3.a
+		if (!innerAlive) {
+			var iter = Call(iterable['[[OpenMethod]]'], iterable['[[Iterable]]']); // step 3.a.i
+			if (Type(iter) !== 'Object') {
+				throw new $TypeError('`Iterator.concat` iterator method did not return an object'); // step 3.a.ii
+			}
+			iteratorRecord = GetIteratorDirect(iter); // step 3.a.iii
+			innerAlive = true; // step 3.a.iv
+		}
+
+		if (innerAlive) { // step 3.a.v
+			var innerValue = IteratorStepValue(iteratorRecord); // step 3.a.v.1
+			if (iteratorRecord['[[Done]]']) { // step 3.a.v.2
+				innerAlive = false; // step 3.a.v.2.a
+			} else { // step 3.a.v.3
+				// 1. Let completion be Completion(Yield(innerValue)).
+				return innerValue; // step 3.a.v.3.a
+			}
+		}
+
+		iterablesIndex += 1;
+		return closure();
+	};
+
 	var closeIfAbrupt = function (abruptCompletion) {
 		if (!(abruptCompletion instanceof CompletionRecord)) {
 			throw new $TypeError('`abruptCompletion` must be a Completion Record');
 		}
-		if (openIters.length > 0) {
+		iterablesIndex = iterables.length;
+		innerAlive = false;
+		if (iteratorRecord) {
 			IteratorCloseAll(
-				openIters,
+				[iteratorRecord],
 				abruptCompletion
 			);
 		}
 	};
 
-	var index = 0;
-	var closure = function () { // step 4
-		if (index < iterables.length) {
-		// forEach(iterables, function (iterable) { // step 4.a
-			var iteratorRecord;
-			if (openIters.length === 0) {
-				var iterable = iterables[index];
-				var iter = Call(iterable['[[OpenMethod]]'], iterable['[[Iterable]]']); // step 4.a.i
-				if (Type(iter) !== 'Object') {
-					closeIfAbrupt(ThrowCompletion(new $TypeError('???'))); // step 4.a.ii
-				}
-				iteratorRecord = GetIteratorDirect(iter); // step 4.a.iii
-				iteratorRecord.obj = iterable['[[Iterable]]'];
-				openIters[openIters.length] = iteratorRecord; // step 4.a.iv
-			} else {
-				iteratorRecord = openIters[0];
-			}
-
-			// var innerAlive = true; // step 4.a.v
-			// while (innerAlive) { // step 4.a.vi
-			{ // eslint-disable-line no-lone-blocks
-				// step 4.a.vi.3.a
-				var innerValue;
-				try {
-					innerValue = IteratorStepValue(iteratorRecord); // step 5.b.ix.4.a
-				} catch (e) {
-					// innerAlive = false;
-					$splice(openIters, $indexOf(openIters, iteratorRecord), 1); // step 4.a.vi.2.a
-					index += 1;
-					closeIfAbrupt(ThrowCompletion(e)); // step 4.a.vi.3.b
-				}
-				if (iteratorRecord['[[Done]]']) {
-					// innerAlive = false;
-					$splice(openIters, $indexOf(openIters, iteratorRecord), 1);
-					index += 1;
-					return closure();
-				}
-				return innerValue; // // step 4.a.vi.3.a
-			}
-		// });
-		}
-
-		// return ReturnCompletion(undefined); // step 4.b
-		return sentinel;
-	};
 	SLOT.set(closure, '[[Sentinel]]', sentinel); // for the userland implementation
 	SLOT.set(closure, '[[CloseIfAbrupt]]', closeIfAbrupt); // for the userland implementation
 
diff --git a/test/Iterator.concat.js b/test/Iterator.concat.js
index 85ba5d6..984a560 100644
--- a/test/Iterator.concat.js
+++ b/test/Iterator.concat.js
@@ -278,6 +278,53 @@ module.exports = {
 				s2t.end();
 			});
 
+			st.test('test262: test/built-ins/Iterator/concat/next-method-returns-non-object', function (s2t) {
+				var nonObjectIterator = {
+					next: function () {
+						return null;
+					}
+				};
+
+				var iterable = {};
+				iterable[Symbol.iterator] = function () {
+					return nonObjectIterator;
+				};
+
+				var iterator = concat(iterable);
+
+				s2t['throws'](function () { iterator.next(); }, TypeError);
+
+				s2t.end();
+			});
+
+			st.test('test262: test/built-ins/Iterator/concat/next-method-returns-throwing-done', { skip: !hasPropertyDescriptors }, function (s2t) {
+				var throwingIterator = {
+					next: function () {
+						var result = { done: null, value: 1 };
+						Object.defineProperty(result, 'done', {
+							get: function () {
+								throw new EvalError();
+							}
+						});
+						return result;
+					},
+					'return': function () {
+						throw new Error();
+					}
+				};
+
+				var iterable = {};
+				iterable[Symbol.iterator] = function () {
+					return throwingIterator;
+				};
+
+				var iterator = concat(iterable);
+
+				s2t['throws'](function () { iterator.next(); }, EvalError);
+
+				s2t.end();
+			});
+
 			st.test('test262: test/built-ins/Iterator/concat/next-method-returns-throwing-value-done', { skip: !hasPropertyDescriptors }, function (s2t) {
 				function ReturnCalledError() {}
 				function ValueGetterError() {}
@@ -312,6 +359,53 @@ module.exports = {
 				s2t.end();
 			});
 
+			st.test('test262: test/built-ins/Iterator/concat/next-method-returns-throwing-value', { skip: !hasPropertyDescriptors }, function (s2t) {
+				var throwingIterator = {
+					next: function () {
+						var result = { value: null, done: false };
+						Object.defineProperty(result, 'value', {
+							get: function () {
+								throw new EvalError();
+							}
+						});
+						return result;
+					},
+					'return': function () {
+						throw new Error();
+					}
+				};
+
+				var iterable = {};
+				iterable[Symbol.iterator] = function () {
+					return throwingIterator;
+				};
+
+				var iterator = concat(iterable);
+
+				s2t['throws'](function () { iterator.next(); }, EvalError);
+
+				s2t.end();
+			});
+
+			st.test('test262: test/built-ins/Iterator/concat/next-method-throws', function (s2t) {
+				var throwingIterator = {
+					next: function () {
+						throw new EvalError();
+					}
+				};
+
+				var iterable = {};
+				iterable[Symbol.iterator] = function () {
+					return throwingIterator;
+				};
+
+				var iterator = concat(iterable);
+
+				s2t['throws'](function () { iterator.next(); }, EvalError);
+
+				s2t.end();
+			});
+
 			st.test('test262: test/built-ins/Iterator/concat/return-is-not-forwarded-after-exhaustion', function (s2t) {
 				var testIterator1 = {
 					next: function () {
@@ -411,6 +505,35 @@ module.exports = {
 				s2t.end();
 			});
 
+			st.test('test262: test/built-ins/Iterator/concat/throws-typeerror-when-generator-is-running-next', function (s2t) {
+				var enterCount = 0;
+
+				var iterator;
+
+				var testIterator1 = {
+					next: function () {
+						enterCount += 1;
+						iterator.next();
+						return { done: false };
+					}
+				};
+
+				var iterable = {};
+				iterable[Symbol.iterator] = function () {
+					return testIterator1;
+				};
+
+				iterator = concat(iterable);
+
+				s2t.equal(enterCount, 0);
+
+				s2t['throws'](function () { iterator.next(); }, TypeError);
+
+				s2t.equal(enterCount, 1);
+
+				s2t.end();
+			});
+
 			st.end();
 		});
 	},