diff --git a/js-programming/bytebeat-render.js b/js-programming/bytebeat-render.js
index 47ffe59..2e24d54 100644
--- a/js-programming/bytebeat-render.js
+++ b/js-programming/bytebeat-render.js
@@ -61,7 +61,14 @@ const generateAudio = t => {
 	return t&t>>8
 }
 
+let lastCorrectSample = 127 // FIXME: guessed value
+
 const constrainValue = sample => {
+	if (isNaN(sample) || sample < 0)
+		sample = lastCorrectSample
+	else
+		lastCorrectSample = sample
+
 	switch (SELECTED_TYPE) {
 	case TYPE_BYTEBEAT:
 		return sample & 255