diff --git a/src/Reflection/SignatureMap/Php8SignatureMapProvider.php b/src/Reflection/SignatureMap/Php8SignatureMapProvider.php
index 44b88c873a..86be50426a 100644
--- a/src/Reflection/SignatureMap/Php8SignatureMapProvider.php
+++ b/src/Reflection/SignatureMap/Php8SignatureMapProvider.php
@@ -207,16 +207,21 @@ private function mergeSignatures(FunctionSignature $nativeSignature, FunctionSig
 		}
 
 		$nativeReturnType = $nativeSignature->getNativeReturnType();
-
-		return new FunctionSignature(
-			$parameters,
-			TypehintHelper::decideType(
+		if ($nativeReturnType instanceof MixedType && !$nativeReturnType->isExplicitMixed()) {
+			$returnType = $functionMapSignature->getReturnType();
+		} else {
+			$returnType = TypehintHelper::decideType(
 				$nativeReturnType,
 				TypehintHelper::decideType(
 					$nativeSignature->getReturnType(),
 					$functionMapSignature->getReturnType()
 				)
-			),
+			);
+		}
+
+		return new FunctionSignature(
+			$parameters,
+			$returnType,
 			$nativeReturnType,
 			$nativeSignature->isVariadic()
 		);
diff --git a/tests/PHPStan/Analyser/NodeScopeResolverTest.php b/tests/PHPStan/Analyser/NodeScopeResolverTest.php
index 94f3acd1a9..e1ed374d22 100644
--- a/tests/PHPStan/Analyser/NodeScopeResolverTest.php
+++ b/tests/PHPStan/Analyser/NodeScopeResolverTest.php
@@ -10526,6 +10526,11 @@ public function dataBugEmptyArray(): array
 		return $this->gatherAssertTypes(__DIR__ . '/data/bug-empty-array.php');
 	}
 
+	public function dataBug4205(): array
+	{
+		return $this->gatherAssertTypes(__DIR__ . '/data/bug-4205.php');
+	}
+
 	/**
 	 * @param string $file
 	 * @return array<string, mixed[]>
@@ -10717,6 +10722,7 @@ private function gatherAssertTypes(string $file): array
 	 * @dataProvider dataBug4207
 	 * @dataProvider dataBug4206
 	 * @dataProvider dataBugEmptyArray
+	 * @dataProvider dataBug4205
 	 * @param string $assertType
 	 * @param string $file
 	 * @param mixed ...$args
diff --git a/tests/PHPStan/Analyser/data/bug-4205.php b/tests/PHPStan/Analyser/data/bug-4205.php
new file mode 100644
index 0000000000..48e88511fd
--- /dev/null
+++ b/tests/PHPStan/Analyser/data/bug-4205.php
@@ -0,0 +1,10 @@
+<?php
+
+namespace Bug4205;
+
+use function PHPStan\Analyser\assertType;
+
+function () {
+	$result = set_error_handler(function() {}, E_ALL);
+	assertType('(callable(): mixed)|null', $result);
+};