diff --git a/libgnucash/app-utils/gnc-optiondb.i b/libgnucash/app-utils/gnc-optiondb.i
index dcf9571360..cfd4ec771f 100644
--- a/libgnucash/app-utils/gnc-optiondb.i
+++ b/libgnucash/app-utils/gnc-optiondb.i
@@ -38,6 +38,9 @@ namespace std {
 
 %enddef
 
+%typemap(in) std::size_t "$1 = scm_to_ulong($input);";
+%typemap(out) std::size_t "$result = scm_from_ulong($1);";
+
  //%module sw_gnc_optiondb
 %{
 #include "gnc-optiondb.h"