diff --git a/gdither.c b/gdither.c
index 74f356f..4a483ae 100644
--- a/gdither.c
+++ b/gdither.c
@@ -342,7 +342,7 @@ void gdither_run(GDither s, unsigned int channel, unsigned int length,
 	for (i=0; (i + pos) < length && i < GDITHER_CONV_BLOCK; i++) {
 	    conv[i] = x[pos + i];
 	}
-	gdither_runf(s, channel, i, conv, ycast + s->channels * step);
+	gdither_runf(s, channel, i, conv, ycast + pos * step);
 	pos += i;
     }
 }