diff --git a/generate_base_client.py b/generate_base_client.py
index 52e98a9bb..decdc8d3c 100755
--- a/generate_base_client.py
+++ b/generate_base_client.py
@@ -9,6 +9,12 @@
 import subprocess
 import sys
 
+def dir_path(string):
+    if os.path.isdir(string):
+        return string
+    else:
+        raise NotADirectoryError(string)
+
 cmdline_desc = """\
 Runs Stone to generate Obj-C types and client for the Dropbox client.
 """
@@ -53,8 +59,8 @@
 )
 _cmdline_parser.add_argument(
     '-fs',
-    '--format-script-dir',
-    type=str,
+    '--format-script-path',
+    type=dir_path,
     help='Path to format script directory.',
 )
 _cmdline_parser.add_argument(
@@ -94,7 +100,7 @@ def main():
         os.path.abspath('Source/ObjectiveDropboxOfficial/Shared/Generated')
     dropbox_pkg_path = args.output_path if args.output_path else dropbox_default_output_path
 
-    dropbox_format_script_dir = args.format_script_dir if args.format_script_dir else \
+    dropbox_format_script_path = args.format_script_path if args.format_script_path else \
         os.path.abspath('Format')
 
     dropbox_format_output_path = args.format_output_path if args.format_output_path else dropbox_src_path
@@ -166,7 +172,7 @@ def main():
         print('Formatting source files')
 
     cmd = ['./format_files.sh', dropbox_format_output_path]
-    o = subprocess.check_output(cmd, cwd=dropbox_format_script_dir)
+    o = subprocess.check_output(cmd, cwd=dropbox_format_script_path)
     if o:
         print('Output:', o)