diff --git a/tools/build_api.py b/tools/build_api.py
index 98809ab0376..65f96ec2a26 100644
--- a/tools/build_api.py
+++ b/tools/build_api.py
@@ -398,8 +398,9 @@ def _fill_header(region_list, current_region):
             else:
                 ih = intelhex_offset(region_dict[data].filename, offset=region_dict[data].start)
             if subtype.startswith("CRCITT32"):
-                fmt = {"CRCITT32be": ">l", "CRCITT32le": "<l"}[subtype]
-                header.puts(start, struct.pack(fmt, zlib.crc32(ih.tobinarray())))
+                fmt = {"CRCITT32be": ">L", "CRCITT32le": "<L"}[subtype]
+                crc_val = zlib.crc32(ih.tobinarray()) & 0xffffffff
+                header.puts(start, struct.pack(fmt, crc_val))
             elif subtype.startswith("SHA"):
                 if subtype == "SHA256":
                     hash = hashlib.sha256()