diff --git a/animations/fpmath_patterns.c b/animations/fpmath_patterns.c
index 0de4142..2b05464 100644
--- a/animations/fpmath_patterns.c
+++ b/animations/fpmath_patterns.c
@@ -448,7 +448,7 @@ static unsigned char fixAnimPlasma(unsigned char const x,
                                    fixp_t const t,
                                    void *const r)
 {
-	assert(x < NUM_COLS);
+	assert(x < (LINEBYTES * 8));
 	assert(y < NUM_ROWS);
 
 	// scaling factor
@@ -524,7 +524,7 @@ static unsigned char fixAnimPsychedelic(unsigned char const x,
                                         fixp_t const t,
                                         void *const r)
 {
-	assert(x < NUM_COLS);
+	assert(x < (LINEBYTES * 8));
 	assert(y < NUM_ROWS);
 	fixp_psychedelic_t *p = (fixp_psychedelic_t *)r;
 
diff --git a/simulator/main.c b/simulator/main.c
index 12f63d8..db499f7 100644
--- a/simulator/main.c
+++ b/simulator/main.c
@@ -28,7 +28,7 @@
 #include "trackball.h"
 
 /** Number of bytes per row. */
-#define LINEBYTES ((NUM_COLS + 1) / 8)
+#define LINEBYTES (((NUM_COLS - 1) / 8) + 1)
 
 /** Fake port for simulating joystick input. */
 volatile unsigned char fakeport;
diff --git a/simulator/winmain.c b/simulator/winmain.c
index dd0c054..e831912 100644
--- a/simulator/winmain.c
+++ b/simulator/winmain.c
@@ -35,7 +35,7 @@
 #include "../display_loop.h"
 
 /** Number of bytes per row. */
-#define LINEBYTES ((NUM_COLS + 1) / 8)
+#define LINEBYTES (((NUM_COLS - 1) / 8) + 1)
 
 /** The width (in pixels) of the margin around a LED. */
 #define LED_MARGIN 1