diff --git a/fb/fbseg.c b/fb/fbseg.c
index 80ce740..6c2c7cf 100644
--- a/fb/fbseg.c
+++ b/fb/fbseg.c
@@ -47,7 +47,7 @@ fbBresSolid (DrawablePtr    pDrawable,
 	     int	    e3,
 	     int	    len)
 {
-    FbStip	*dst;
+    FbStip	*dst, *start, *end;
     FbStride	dstStride;
     int		dstBpp;
     int		dstXoff, dstYoff;
@@ -56,12 +56,15 @@ fbBresSolid (DrawablePtr    pDrawable,
     FbStip	xor = (FbStip) pPriv->xor;
     FbStip	mask, mask0;
     FbStip	bits;
-    
+
     fbGetStipDrawable (pDrawable, dst, dstStride, dstBpp, dstXoff, dstYoff);
+    start = dst;
+    end = dst + dstStride * pDrawable->height;
     dst += ((y1 + dstYoff) * dstStride);
     x1 = (x1 + dstXoff) * dstBpp;
     dst += x1 >> FB_STIP_SHIFT;
     x1 &= FB_STIP_MASK;
+    assert (dst >= start && dst < end);
     mask0 = FbStipMask(0, dstBpp);
     mask = FbStipRight (mask0, x1);
     if (signdx < 0)
@@ -77,6 +80,7 @@ fbBresSolid (DrawablePtr    pDrawable,
 	    mask = fbBresShiftMask(mask,signdx,dstBpp);
 	    if (!mask)
 	    {
+		assert (dst >= start && dst < end);
 		WRITE(dst, FbDoMaskRRop (READ(dst), and, xor, bits));
 		bits = 0;
 		dst += signdx;
@@ -85,19 +89,23 @@ fbBresSolid (DrawablePtr    pDrawable,
 	    e += e1;
 	    if (e >= 0)
 	    {
+		assert (dst >= start && dst < end);
 		WRITE(dst, FbDoMaskRRop (READ(dst), and, xor, bits));
 		bits = 0;
 		dst += dstStride;
 		e += e3;
 	    }
 	}
-	if (bits)
+	if (bits) {
+	    assert (dst >= start && dst < end);
 	    WRITE(dst, FbDoMaskRRop (READ(dst), and, xor, bits));
+	}
     }
     else
     {
 	while (len--)
 	{
+	    assert (dst >= start && dst < end);
 	    WRITE(dst, FbDoMaskRRop (READ(dst), and, xor, mask));
 	    dst += dstStride;
 	    e += e1;