diff --git a/FM.cpp b/FM.cpp
index 5c882d0..7695e81 100644
--- a/FM.cpp
+++ b/FM.cpp
@@ -131,9 +131,11 @@ void CFM::process()
     return;
 
   uint16_t space = io.getSpace();
-  if (space == 0U)
+  if (space < 3U)
     return;
 
+  space -= 2U;
+
   if (space < length)
     length = space;