diff --git a/techlibs/common/synth.cc b/techlibs/common/synth.cc
index e5013678a..f604b67c9 100644
--- a/techlibs/common/synth.cc
+++ b/techlibs/common/synth.cc
@@ -151,7 +151,7 @@ struct SynthPass : public ScriptPass {
 				flatten = true;
 				continue;
 			}
-			if (args[argidx] == "-lut") {
+			if (args[argidx] == "-lut" && argidx + 1 < args.size()) {
 				lut = atoi(args[++argidx].c_str());
 				continue;
 			}