diff --git a/tools/build_api.py b/tools/build_api.py
index 4a3de865d3..0f25ffa775 100644
--- a/tools/build_api.py
+++ b/tools/build_api.py
@@ -1357,4 +1357,4 @@ def merge_build_data(filename, toolchain_report, app_type):
                     if 'type' not in build[0]:
                         build[0]['type'] = app_type
                     build_data['builds'].insert(0, build[0])
-    dump(build_data, open(filename, "wb"), indent=4, separators=(',', ': '))
+    dump(build_data, open(filename, "w"), indent=4, separators=(',', ': '))