diff --git a/firmware/common/fifo.hpp b/firmware/common/fifo.hpp
index f7a490c7..0834d20f 100644
--- a/firmware/common/fifo.hpp
+++ b/firmware/common/fifo.hpp
@@ -59,7 +59,7 @@ public:
 	}
 
 	bool is_full() const {
-		return len() > mask();
+		return unused() == 0;
 	}
 /*
 	bool in(const T& val) {