diff --git a/lib/Figments/Display.cpp b/lib/Figments/Display.cpp
index d92f283..a933080 100644
--- a/lib/Figments/Display.cpp
+++ b/lib/Figments/Display.cpp
@@ -43,6 +43,7 @@ CRGB&
 Display::pixelAt(int idx)
 {
     const int kx = idx % pixelCount();
+    assert(abs(kx) < pixelCount());
     if (kx < 0) {
         return m_pixels[pixelCount() + 1 + kx];
     } else {