diff --git a/tools/build_api.py b/tools/build_api.py
index f0c2c60d53..3589a95242 100644
--- a/tools/build_api.py
+++ b/tools/build_api.py
@@ -377,7 +377,7 @@ def _fill_header(region_list, current_region):
         elif type == "timestamp":
             fmt = {"32le": "<L", "64le": "<Q",
                    "32be": ">L", "64be": ">Q"}[subtype]
-            header.puts(start, struct.pack(fmt, time()))
+            header.puts(start, struct.pack(fmt, int(time())))
         elif type == "size":
             fmt = {"32le": "<L", "64le": "<Q",
                    "32be": ">L", "64be": ">Q"}[subtype]