diff --git a/litex/build/xilinx/vivado.py b/litex/build/xilinx/vivado.py
index 01fd8075b..f22b44c92 100644
--- a/litex/build/xilinx/vivado.py
+++ b/litex/build/xilinx/vivado.py
@@ -272,7 +272,7 @@ class XilinxVivadoToolchain:
             self.false_paths.add((from_, to))
 
 def vivado_build_args(parser):
-    parser.add_argument("--synth-mode", default=None, help="synthesis mode (vivado or yosys)")
+    parser.add_argument("--synth-mode", default="vivado", help="synthesis mode (vivado or yosys)")
 
 
 def vivado_build_argdict(args):