diff --git a/src/Type/Php/FilterFunctionReturnTypeHelper.php b/src/Type/Php/FilterFunctionReturnTypeHelper.php
index 1e920328bd..98834338b5 100644
--- a/src/Type/Php/FilterFunctionReturnTypeHelper.php
+++ b/src/Type/Php/FilterFunctionReturnTypeHelper.php
@@ -191,7 +191,7 @@ public function getType(Type $inputType, ?Type $filterType, ?Type $flagsType): T
 		}
 
 		if ($exactType === null || $hasOptions->maybe() || (!$inputType->equals($type) && $inputType->isSuperTypeOf($type)->yes())) {
-			if ($defaultType->isSuperTypeOf($type)->no()) {
+			if (!$defaultType->isSuperTypeOf($type)->yes()) {
 				$type = TypeCombinator::union($type, $defaultType);
 			}
 		}
@@ -495,11 +495,17 @@ private function hasFlag(string $flagName, ?Type $flagsType): TrinaryLogic
 		}
 
 		$type = $this->getFlagsValue($flagsType);
-		if (!$type instanceof ConstantIntegerType) {
+		$scalarValues = $type->getConstantScalarValues();
+		if ($scalarValues === []) {
 			return TrinaryLogic::createMaybe();
 		}
 
-		return TrinaryLogic::createFromBoolean(($type->getValue() & $flag) === $flag);
+		return TrinaryLogic::lazyExtremeIdentity(
+			$scalarValues,
+			static fn (bool|string|int|float|null $scalar): TrinaryLogic => TrinaryLogic::createFromBoolean(
+				(((int) $scalar) & $flag) === $flag,
+			),
+		);
 	}
 
 	private function getFlagsValue(Type $exprType): Type
diff --git a/tests/PHPStan/Analyser/nsrt/filter-var.php b/tests/PHPStan/Analyser/nsrt/filter-var.php
index 1935c117aa..34d23f9c8f 100644
--- a/tests/PHPStan/Analyser/nsrt/filter-var.php
+++ b/tests/PHPStan/Analyser/nsrt/filter-var.php
@@ -169,4 +169,15 @@ public function scalars(bool $bool, float $float, int $int, string $string, int
 		assertType("''", filter_var(null));
 	}
 
+	public function randomFlag($mixed, bool $bool) {
+
+		assertType('int|false|null', filter_var($mixed, FILTER_VALIDATE_INT, [
+			'flags' => $bool ? FILTER_NULL_ON_FAILURE : FILTER_FLAG_NONE,
+		]));
+
+		assertType('bool|null', filter_var($mixed, FILTER_VALIDATE_BOOLEAN, [
+			'flags' => $bool ? FILTER_NULL_ON_FAILURE : FILTER_FLAG_NONE,
+		]));
+	}
+
 }