diff --git a/compiler/pipes/calc-func-dep.cpp b/compiler/pipes/calc-func-dep.cpp
index b6adbfa017..5bfdeb7f88 100644
--- a/compiler/pipes/calc-func-dep.cpp
+++ b/compiler/pipes/calc-func-dep.cpp
@@ -33,6 +33,11 @@ VertexPtr CalcFuncDepPass::on_enter_vertex(VertexPtr vertex) {
   }
 
   if (auto instanceof = vertex.try_as<op_instanceof>()) {
+    // mark_as_used() is required here for the same reason as in op_catch below:
+    // a class used only in instanceof may never be instantiated elsewhere,
+    // so without this call the code-gen pass would crash trying to include
+    // a header that was never generated (see #68)
+    instanceof->derived_class->mark_as_used();
     current_function->class_dep.insert(instanceof->derived_class);
   }
 
diff --git a/tests/phpt/interfaces/135_instanceof_unused_class.php b/tests/phpt/interfaces/135_instanceof_unused_class.php
new file mode 100644
index 0000000000..2f064eb3c5
--- /dev/null
+++ b/tests/phpt/interfaces/135_instanceof_unused_class.php
@@ -0,0 +1,22 @@
+@ok
+<?php
+
+// Regression test for https://github.com/VKCOM/kphp/issues/68
+// A class that appears only inside instanceof (never directly instantiated)
+// must compile without crashing with "Can't find header".
+
+class Bar {}
+class Foo extends Bar {}
+
+function check_foo(Bar $obj): bool {
+  return $obj instanceof Foo;
+}
+
+// Foo is never instantiated — only used in instanceof.
+// Before the fix, kphp crashed trying to include cl/C@Foo.h
+// which was never generated because Foo was never mark_as_used().
+$bar = new Bar();
+var_dump(check_foo($bar));   // bool(false)
+
+$foo = new Foo();
+var_dump(check_foo($foo));   // bool(true)
diff --git a/tests/phpt/interfaces/136_instanceof_only_in_condition.php b/tests/phpt/interfaces/136_instanceof_only_in_condition.php
new file mode 100644
index 0000000000..d4e96f399a
--- /dev/null
+++ b/tests/phpt/interfaces/136_instanceof_only_in_condition.php
@@ -0,0 +1,30 @@
+@ok
+<?php
+
+// Additional regression for #68: instanceof with a class that is
+// referenced nowhere else in the translation unit.
+
+interface Shape {}
+
+class Circle implements Shape {
+  public float $radius;
+  public function __construct(float $r) { $this->radius = $r; }
+}
+
+class Square implements Shape {
+  public float $side;
+  public function __construct(float $s) { $this->side = $s; }
+}
+
+// Triangle is declared but never instantiated — only appears in instanceof.
+class Triangle implements Shape {}
+
+function describe(Shape $s): string {
+  if ($s instanceof Circle) return 'circle';
+  if ($s instanceof Square) return 'square';
+  if ($s instanceof Triangle) return 'triangle';  // Triangle: only in instanceof
+  return 'unknown';
+}
+
+var_dump(describe(new Circle(1.0)));   // string(6) "circle"
+var_dump(describe(new Square(2.0)));   // string(6) "square"