diff --git a/tools/releasetools/add_img_to_target_files.py b/tools/releasetools/add_img_to_target_files.py
index 934b73f517..8d660f88b9 100644
--- a/tools/releasetools/add_img_to_target_files.py
+++ b/tools/releasetools/add_img_to_target_files.py
@@ -1083,10 +1083,15 @@ def AddImagesToTargetFiles(filename):
       ("system_dlkm", has_system_dlkm, AddSystemDlkm, []),
       ("system_other", has_system_other, AddSystemOther, []),
   )
-
-  with ThreadPoolExecutor(max_workers=len(add_partition_calls)) as executor:
-    for future in [executor.submit(add_partition, *call) for call in add_partition_calls]:
-      future.result()
+  # If output_zip exists, each add_partition_calls writes bytes to the same output_zip,
+  # which is not thread-safe. So, run them in serial if output_zip exists.
+  if output_zip:
+    for call in add_partition_calls:
+      add_partition(*call)
+  else:
+    with ThreadPoolExecutor(max_workers=len(add_partition_calls)) as executor:
+      for future in [executor.submit(add_partition, *call) for call in add_partition_calls]:
+        future.result()
 
   AddApexInfo(output_zip)