diff --git a/gc_heap.c b/gc_heap.c
index 07b4a583..c943c9f1 100644
--- a/gc_heap.c
+++ b/gc_heap.c
@@ -608,7 +608,7 @@ sexp sexp_load_image (const char* filename, off_t offset, sexp_uint_t heap_free_
     goto done;
   }
   if (offset > 0 && fseek(fp, offset, SEEK_SET) < 0) {
-    snprintf(gc_heap_err_str, ERR_STR_SIZE, "couldn't seek to image offset: %s -> %lld: %s\n", filename, offset, strerror(errno));
+    snprintf(gc_heap_err_str, ERR_STR_SIZE, "couldn't seek to image offset: %s -> "PRIoff": %s\n", filename, offset, strerror(errno));
     goto done;
   }
 
diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index c2c22d0c..5d467460 100755
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -256,6 +256,12 @@ typedef short sexp_int32_t;
 # endif
 #endif
 
+#ifdef __APPLE__
+#define PRIoff "%lld"
+#else
+#define PRIoff "%ld"
+#endif
+
 #if SEXP_USE_LONG_PROCEDURE_ARGS
 typedef int sexp_proc_num_args_t;
 #else