diff --git a/src/engine/zsm.cpp b/src/engine/zsm.cpp
index 6f1e02df1..7768e6590 100644
--- a/src/engine/zsm.cpp
+++ b/src/engine/zsm.cpp
@@ -207,7 +207,7 @@ void ZSM::flushWrites() {
 void ZSM::flushTicks() {
   while (ticks > ZSM_DELAY_MAX) {
 	logD("ZSM: write delay %d (max)",ZSM_DELAY_MAX);
-	w->writeC(ZSM_DELAY_CMD+ZSM_DELAY_MAX);
+	w->writeC((signed char)(ZSM_DELAY_CMD+ZSM_DELAY_MAX));
 	ticks -= ZSM_DELAY_MAX;
   }
   if (ticks>0) {