diff --git a/src/request-parameters/RequestParametersBase.php b/src/request-parameters/RequestParametersBase.php
index a4a5d57..01d894d 100644
--- a/src/request-parameters/RequestParametersBase.php
+++ b/src/request-parameters/RequestParametersBase.php
@@ -64,7 +64,7 @@ classname<T> $class,
     $spec = $specs[$name];
     invariant(
       /* HH_FIXME[4162] need reified generics */
-      $spec instanceof $class,
+      \is_a($spec, $class),
       'Expected %s to be a %s, got %s',
       $name,
       $class,
diff --git a/src/uri-patterns/UriBuilderBase.php b/src/uri-patterns/UriBuilderBase.php
index e157a77..055e496 100644
--- a/src/uri-patterns/UriBuilderBase.php
+++ b/src/uri-patterns/UriBuilderBase.php
@@ -74,7 +74,7 @@ classname<TypedUriParameter<T>> $parameter_type,
       \implode(', ', $this->parameters->keys()->map($x ==> "'".$x."'")),
     );
     invariant(
-      $part instanceof $parameter_type,
+      \is_a($part, $parameter_type),
       'Expected %s to be a %s, got a %s',
       $name,
       $parameter_type,