diff --git a/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc b/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc
index 38a9a181a18..04b0b580b90 100644
--- a/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc
+++ b/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc
@@ -1555,6 +1555,13 @@ Fragment StreamingFlowGraphBuilder::BuildFunctionBody(
   // initializers will be visible inside the entire body of the constructor.
   // We should make a separate scope for them.
   if (constructor) {
+    // PatchClass with different kernel_data means we are building a constructor
+    // for some old class. That is not supported and should not be happening as
+    // old classes are gone after hot reload, we don't have reference to old
+    // class's fields.
+    ASSERT(!Object::Handle(dart_function.RawOwner()).IsPatchClass() ||
+           (Library::Handle(Class::Handle(dart_function.Owner()).library())
+                .kernel_data() == dart_function.KernelData()));
     body += BuildInitializers(Class::Handle(Z, dart_function.Owner()));
   }