--- a/src/lib/cache.c	Tue Dec 29 02:11:27 2009 +0200
+++ b/src/lib/cache.c	Tue Dec 29 12:29:50 2009 +0200
@@ -444,7 +444,7 @@
     if (ti->x + ti->width > cache->header->width)
         clip_x = cache->header->width;
     else
-        clip_y = ti->x + ti->width;
+        clip_x = ti->x + ti->width;
     
     // figure out if the tile clips over the bottom edge
     // XXX: use min()
@@ -491,6 +491,13 @@
 {
     int err;
 
+    // check within bounds
+    if (ti->x >= cache->header->width || ti->y >= cache->header->height) {
+        // completely outside
+        errno = EINVAL;
+        return -1;
+    }
+
     // ensure open
     if (pt_cache_open(cache))
         return -1;

--- a/src/lib/pngtile.h	Tue Dec 29 02:11:27 2009 +0200
+++ b/src/lib/pngtile.h	Tue Dec 29 12:29:50 2009 +0200
@@ -51,7 +51,11 @@
     size_t width, height;
 };
 
-/** Info for image tile */
+/**
+ * Info for image tile 
+ *
+ * The tile may safely overlap with the edge of the image, but it should not be entirely outside of the image
+ */
 struct pt_tile_info {
     /** Dimensions of output image */
     size_t width, height;