diff --git a/siso.py b/siso.py
index 35a3f0f8c6..328386c182 100644
--- a/siso.py
+++ b/siso.py
@@ -114,6 +114,11 @@ def main(args):
             if siso_path and os.path.isfile(siso_path):
                 checkOutdir(args[1:])
                 return subprocess.call([siso_path] + args[1:], env=env)
+        print(
+            'depot_tools/siso.py: Could not find siso in third_party/siso '
+            'of the current project. Did you run gclient sync?',
+            file=sys.stderr)
+        return 1
     if siso_override_path:
         return subprocess.call([siso_override_path] + args[1:])