diff --git a/scripts/mbedtls_dev/test_generation.py b/scripts/mbedtls_dev/test_generation.py
index 81af7ba..87cb43e 100644
--- a/scripts/mbedtls_dev/test_generation.py
+++ b/scripts/mbedtls_dev/test_generation.py
@@ -137,7 +137,7 @@
 class TestGenerator:
     """Generate test cases and write to data files."""
     def __init__(self, options) -> None:
-        self.test_suite_directory = getattr(options, 'directory')
+        self.test_suite_directory = getattr(options, 'directory', 'tests/suites')
         # Update `targets` with an entry for each child class of BaseTarget.
         # Each entry represents a file generated by the BaseTarget framework,
         # and enables generating the .data files using the CLI.
@@ -179,8 +179,12 @@
                         help='List available targets and exit')
     parser.add_argument('--list-for-cmake', action='store_true',
                         help='Print \';\'-separated list of available targets and exit')
-    parser.add_argument('--directory', default="tests/suites", metavar='DIR',
+    parser.add_argument('--directory', metavar='DIR',
                         help='Output directory (default: tests/suites)')
+    # The `--directory` option is interpreted relative to the directory from
+    # which the script is invoked, but the default is relative to the root of
+    # the mbedtls tree. The default should not be set above, but instead after
+    # `build_tree.chdir_to_root()` is called.
     parser.add_argument('targets', nargs='*', metavar='TARGET',
                         help='Target file to generate (default: all; "-": none)')
     options = parser.parse_args(args)