diff --git a/src/Type/WebMozartAssert/AssertTypeSpecifyingExtension.php b/src/Type/WebMozartAssert/AssertTypeSpecifyingExtension.php
index 3cdb39a..ae7bf6a 100644
--- a/src/Type/WebMozartAssert/AssertTypeSpecifyingExtension.php
+++ b/src/Type/WebMozartAssert/AssertTypeSpecifyingExtension.php
@@ -18,6 +18,7 @@
 use PHPStan\Type\MixedType;
 use PHPStan\Type\ObjectType;
 use PHPStan\Type\StaticMethodTypeSpecifyingExtension;
+use PHPStan\Type\StringType;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
 use PHPStan\Type\TypeUtils;
@@ -302,10 +303,26 @@ private static function getExpressionResolvers(): array
 				},
 				'implementsInterface' => function (Scope $scope, Arg $expr, Arg $class): ?\PhpParser\Node\Expr {
 					$classType = $scope->getType($class->value);
+					$exprType = $scope->getType($expr->value);
 					if (!$classType instanceof ConstantStringType) {
 						return null;
 					}
 
+					if ($exprType instanceof StringType) {
+						return new \PhpParser\Node\Expr\FuncCall(
+							new \PhpParser\Node\Name('in_array'),
+							[
+								$class,
+								new \PhpParser\Node\Arg(
+									new \PhpParser\Node\Expr\FuncCall(
+										new \PhpParser\Node\Name('class_implements'),
+										[$expr]
+									)
+								),
+							]
+						);
+					}
+
 					return new \PhpParser\Node\Expr\Instanceof_(
 						$expr->value,
 						new \PhpParser\Node\Name($classType->getValue())
diff --git a/tests/Type/WebMozartAssert/data/data.php b/tests/Type/WebMozartAssert/data/data.php
index 5c92662..5dc1373 100644
--- a/tests/Type/WebMozartAssert/data/data.php
+++ b/tests/Type/WebMozartAssert/data/data.php
@@ -120,6 +120,8 @@ public function doFoo($a, $b, array $c, iterable $d, $e, $f, $g, $h, $i, $j, $k,
 
         Assert::implementsInterface($ae, Baz::class);
         $ae;
+
+        Assert::implementsInterface(Bar::class, Foo::class);
     }
 
 }