diff --git a/src/Type/Constant/ConstantArrayTypeBuilder.php b/src/Type/Constant/ConstantArrayTypeBuilder.php
index 525b65caed..a306833e8d 100644
--- a/src/Type/Constant/ConstantArrayTypeBuilder.php
+++ b/src/Type/Constant/ConstantArrayTypeBuilder.php
@@ -199,8 +199,6 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $opt
 				return;
 			}
 
-			$this->isList = TrinaryLogic::createNo();
-
 			$scalarTypes = $offsetType->getConstantScalarTypes();
 			if (count($scalarTypes) === 0) {
 				$integerRanges = TypeUtils::getIntegerRanges($offsetType);
@@ -257,6 +255,8 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $opt
 					return;
 				}
 			}
+
+			$this->isList = TrinaryLogic::createNo();
 		}
 
 		if ($offsetType === null) {
diff --git a/tests/PHPStan/Analyser/nsrt/array-is-list-offset.php b/tests/PHPStan/Analyser/nsrt/array-is-list-offset.php
new file mode 100644
index 0000000000..911490fac7
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/array-is-list-offset.php
@@ -0,0 +1,19 @@
+<?php
+
+namespace ArrayIsListUnset;
+
+use function PHPStan\Testing\assertType;
+
+class Foo {
+	/**
+	 * @param array{bool, bool, bool} $array
+	 * @param int<0, 1> $key
+	 */
+	public function test(array $array, int $key) {
+		assertType('int<0, 1>', $key);
+		assertType('true', array_is_list($array));
+
+		$array[$key] = false;
+		assertType('true', array_is_list($array));
+	}
+}
diff --git a/tests/PHPStan/Type/Constant/ConstantArrayTypeBuilderTest.php b/tests/PHPStan/Type/Constant/ConstantArrayTypeBuilderTest.php
index 0b8bff2efb..2d1aaa237c 100644
--- a/tests/PHPStan/Type/Constant/ConstantArrayTypeBuilderTest.php
+++ b/tests/PHPStan/Type/Constant/ConstantArrayTypeBuilderTest.php
@@ -5,6 +5,7 @@
 use PHPStan\Type\BooleanType;
 use PHPStan\Type\NullType;
 use PHPStan\Type\StringType;
+use PHPStan\Type\TypeCombinator;
 use PHPStan\Type\VerbosityLevel;
 use PHPUnit\Framework\TestCase;
 
@@ -128,4 +129,37 @@ public function testIsList(): void
 		$this->assertFalse($builder->isList());
 	}
 
+	public function testIsListWithUnion(): void
+	{
+		$builder = ConstantArrayTypeBuilder::createEmpty();
+
+		$builder->setOffsetValueType(null, new ConstantIntegerType(0));
+		$this->assertTrue($builder->isList());
+
+		$builder->setOffsetValueType(new ConstantIntegerType(0), new NullType());
+		$this->assertTrue($builder->isList());
+
+		$builder->setOffsetValueType(new ConstantIntegerType(1), new NullType());
+		$this->assertTrue($builder->isList());
+
+		$builder->setOffsetValueType(new ConstantIntegerType(2), new NullType());
+		$this->assertTrue($builder->isList());
+
+		$oneOrZero = TypeCombinator::union(
+			new ConstantIntegerType(0),
+			new ConstantIntegerType(1),
+		);
+
+		$builder->setOffsetValueType($oneOrZero, new NullType());
+		$this->assertTrue($builder->isList());
+
+		$oneOrFour = TypeCombinator::union(
+			new ConstantIntegerType(1),
+			new ConstantIntegerType(4),
+		);
+
+		$builder->setOffsetValueType($oneOrFour, new NullType());
+		$this->assertFalse($builder->isList());
+	}
+
 }