diff --git a/src/Type/WebMozartAssert/AssertTypeSpecifyingExtension.php b/src/Type/WebMozartAssert/AssertTypeSpecifyingExtension.php
index 600ac33..b4c4f8b 100644
--- a/src/Type/WebMozartAssert/AssertTypeSpecifyingExtension.php
+++ b/src/Type/WebMozartAssert/AssertTypeSpecifyingExtension.php
@@ -410,15 +410,21 @@ private function getExpressionResolvers(): array
 				},
 				'isInstanceOf' => static function (Scope $scope, Arg $expr, Arg $class): ?Expr {
 					$classType = $scope->getType($class->value);
-					$classNameType = $classType->getObjectTypeOrClassStringObjectType();
-					$classNames = $classNameType->getObjectClassNames();
-					if (count($classNames) !== 1) {
+					$classNameStrings = $classType->getConstantStrings();
+					if (count($classNameStrings) !== 1) {
+						$classNames = $classType->getObjectClassNames();
+						if (count($classNames) === 1) {
+							return new Instanceof_(
+								$expr->value,
+								new Name($classNames[0])
+							);
+						}
 						return null;
 					}
 
 					return new Instanceof_(
 						$expr->value,
-						new Name($classNames[0])
+						new Name($classNameStrings[0]->getValue())
 					);
 				},
 				'isInstanceOfAny' => function (Scope $scope, Arg $expr, Arg $classes): ?Expr {
diff --git a/tests/Type/WebMozartAssert/data/impossible-check.php b/tests/Type/WebMozartAssert/data/impossible-check.php
index 8175e26..271efc3 100644
--- a/tests/Type/WebMozartAssert/data/impossible-check.php
+++ b/tests/Type/WebMozartAssert/data/impossible-check.php
@@ -111,6 +111,14 @@ public function implementsInterface($a, string $b, $c): void
 		Assert::implementsInterface($c, self::class);
 	}
 
+	/**
+	 * @param class-string<\Exception> $name
+	 */
+	public function testInstanceOfClassString(\Exception $e, string $name): void
+	{
+		Assert::isInstanceOf($e, $name);
+	}
+
 }
 
 interface Bar {};