diff --git a/src/goto-programs/interpreter_class.h b/src/goto-programs/interpreter_class.h
index ccc4730fba0..04b3d32446d 100644
--- a/src/goto-programs/interpreter_class.h
+++ b/src/goto-programs/interpreter_class.h
@@ -273,7 +273,7 @@ class interpretert:public messaget
   dynamic_typest dynamic_types;
   int num_dynamic_objects;
   mp_integer stack_depth;
-  int thread_id;
+  unsigned thread_id;
 
   bool evaluate_boolean(const exprt &expr)
   {