diff --git a/compiler/pipes/inline-simple-functions.cpp b/compiler/pipes/inline-simple-functions.cpp
index f6bbaefc8..45fa97ca9 100644
--- a/compiler/pipes/inline-simple-functions.cpp
+++ b/compiler/pipes/inline-simple-functions.cpp
@@ -4,6 +4,7 @@
 
 #include "compiler/pipes/inline-simple-functions.h"
 
+#include "compiler/data/class-data.h"
 #include "compiler/data/src-file.h"
 #include "compiler/data/var-data.h"
 #include "compiler/inferring/public.h"
@@ -107,6 +108,17 @@ bool InlineSimpleFunctions::check_function(FunctionPtr function) const {
          !function->kphp_lib_export;
 }
 
+
+void InlineSimpleFunctions::on_start() {
+  if (auto klass = current_function->class_id) {
+    if (klass->internal_interface) {
+      inline_is_possible_ = false;
+    }
+  }
+  return FunctionPassBase::on_start();
+}
+
+
 void InlineSimpleFunctions::on_finish() {
   if (inline_is_possible_) {
     current_function->is_inline = true;
diff --git a/compiler/pipes/inline-simple-functions.h b/compiler/pipes/inline-simple-functions.h
index da5aedc44..c5f88a9f5 100644
--- a/compiler/pipes/inline-simple-functions.h
+++ b/compiler/pipes/inline-simple-functions.h
@@ -21,5 +21,6 @@ class InlineSimpleFunctions final : public FunctionPassBase {
   VertexPtr on_exit_vertex(VertexPtr root) final;
   bool user_recursion(VertexPtr) final;
   bool check_function(FunctionPtr function) const final;
+  void on_start() final;
   void on_finish() final;
 };