diff --git a/libguile/_scm.h b/libguile/_scm.h
index a6316c0a8..ea654ad39 100644
--- a/libguile/_scm.h
+++ b/libguile/_scm.h
@@ -159,12 +159,13 @@
 #define scm_to_off_t_or_off64_t         CHOOSE_LARGEFILE(scm_to_off_t,scm_to_int64)
 
 #if SIZEOF_OFF_T == 4
-#define scm_to_off_t    scm_to_int32
-#define scm_from_off_t  scm_from_int32
-#endif
-#if SIZEOF_OFF_T == 8
-#define scm_to_off_t    scm_to_int64
-#define scm_from_off_t  scm_from_int64
+#  define scm_to_off_t    scm_to_int32
+#  define scm_from_off_t  scm_from_int32
+#elif SIZEOF_OFF_T == 8
+#  define scm_to_off_t    scm_to_int64
+#  define scm_from_off_t  scm_from_int64
+#else
+#  error sizeof(off_t) is not 4 or 8.
 #endif