diff --git a/sim_gen/ISA/isa.rb b/sim_gen/ISA/isa.rb
index bed0076..b04ca4d 100644
--- a/sim_gen/ISA/isa.rb
+++ b/sim_gen/ISA/isa.rb
@@ -102,6 +102,7 @@ def generate_isa_header(input_ir)
 "#ifndef GENERATED_#{input_ir[:isa_name].upcase}_ISA_HH_INCLUDED
 #define GENERATED_#{input_ir[:isa_name].upcase}_ISA_HH_INCLUDED
 
+#include <cstddef>
 #include <cstdint>
 
 namespace prot::isa {