diff --git a/wrlib/convert.c b/wrlib/convert.c
index 05a82487..8b5d3c7a 100644
--- a/wrlib/convert.c
+++ b/wrlib/convert.c
@@ -778,6 +778,10 @@ static RXImage *image2GrayScale(RContext * ctx, RImage * image)
 			RDestroyXImage(ctx, ximg);
 			return NULL;
 		}
+
+		memset(gerr, 0, (image->width + 2) * sizeof(short));
+		memset(ngerr, 0, (image->width + 2) * sizeof(short));
+
 		for (x = 0, y = 0; x < image->width; x++, y += channels) {
 			gerr[x] = (ptr[y] * 30 + ptr[y + 1] * 59 + ptr[y + 2] * 11) / 100;
 		}