diff --git a/src/Rules/Comparison/ImpossibleCheckTypeHelper.php b/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
index b0e71448b5..0551a2b2d1 100644
--- a/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
+++ b/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
@@ -153,7 +153,7 @@ public function findSpecifiedType(
 							foreach ($haystackArrayTypes as $haystackArrayType) {
 								if ($haystackArrayType instanceof ConstantArrayType) {
 									foreach ($haystackArrayType->getValueTypes() as $i => $haystackArrayValueType) {
-										if ($haystackArrayType->isOptionalKey($i)) {
+										if (count($haystackArrayValueType->getFiniteTypes()) > 1 || $haystackArrayType->isOptionalKey($i)) {
 											continue;
 										}
 
diff --git a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
index f83bd6f26f..726954bb66 100644
--- a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
+++ b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
@@ -995,8 +995,16 @@ public function testBug8818(): void
 
 	public function testBug12755(): void
 	{
+		$tipText = 'Because the type is coming from a PHPDoc, you can turn off this check by setting <fg=cyan>treatPhpDocTypesAsCertain: false</> in your <fg=cyan>%configurationFile%</>.';
+
 		$this->treatPhpDocTypesAsCertain = true;
-		$this->analyse([__DIR__ . '/data/bug-12755.php'], []);
+		$this->analyse([__DIR__ . '/data/bug-12755.php'], [
+			[
+				'Call to function in_array() with arguments null, array{key1: bool|null, key2: null} and true will always evaluate to true.',
+				51,
+				$tipText,
+			],
+		]);
 	}
 
 	public function testBug12412(): void
diff --git a/tests/PHPStan/Rules/Comparison/data/bug-12755.php b/tests/PHPStan/Rules/Comparison/data/bug-12755.php
index 6df2876e3b..99bd5faac5 100644
--- a/tests/PHPStan/Rules/Comparison/data/bug-12755.php
+++ b/tests/PHPStan/Rules/Comparison/data/bug-12755.php
@@ -2,6 +2,13 @@
 
 namespace Bug12755;
 
+class MyEnum
+{
+	public const ONE = 'one';
+	public const TWO = 'two';
+	public const THREE = 'three';
+}
+
 class HelloWorld
 {
 	/**
@@ -32,4 +39,52 @@ public function testBool(array $myArray): ?\stdClass
 
 		return (object) $myArray;
 	}
+
+	/**
+	 * @param array{
+	 *     key1: ?bool,
+	 *     key2: null,
+	 * } $myArray
+	 */
+	public function testNull(array $myArray): ?\stdClass
+	{
+	    if (\in_array(null, $myArray, true)) {
+	        return null;
+	    }
+
+		return (object) $myArray;
+	}
+
+    /**
+     * @param list<MyEnum::*> $stack
+     */
+    public function testEnum(array $stack): bool
+    {
+        return count($stack) === 1 && in_array(MyEnum::ONE, $stack, true);
+    }
+
+	/**
+	 * @param array{1|2|3} $stack
+	 * @param array{1|2|3, 1|2|3} $stack2
+	 * @param array{1|2|3, 2|3} $stack3
+	 * @param array{a?: 1, b: 2|3} $stack4
+	 * @param array{a?: 1} $stack5
+	 */
+	public function sayHello(array $stack, array $stack2, array $stack3, array $stack4, array $stack5): void
+	{
+		if (in_array(1, $stack, true)) {
+		}
+
+		if (in_array(1, $stack2, true)) {
+		}
+
+		if (in_array(1, $stack3, true)) {
+		}
+
+		if (in_array(1, $stack4, true)) {
+		}
+		
+		if (in_array(1, $stack5, true)) {
+		}
+	}
 }