diff --git a/src/programs.c b/src/programs.c
index 797cebeea..44bbed1b6 100644
--- a/src/programs.c
+++ b/src/programs.c
@@ -134,7 +134,7 @@ SCM_DEFINE (scm_program_base, "program-base", 1, 0, 0,
 {
   SCM_VALIDATE_PROGRAM (1, program);
 
-  return scm_long2num ((long) SCM_PROGRAM_DATA (program)->base);
+  return scm_ulong2num ((unsigned long) SCM_PROGRAM_DATA (program)->base);
 }
 #undef FUNC_NAME