diff --git a/libvips/conversion/cache.c b/libvips/conversion/cache.c
index e91d6123..52f31790 100644
--- a/libvips/conversion/cache.c
+++ b/libvips/conversion/cache.c
@@ -159,7 +159,10 @@ vips_cache_init( VipsCache *cache )
  * documentation for that operation for details. 
  *
  * It uses a set of background threads to calculate pixels and the various
- * active cache operations coordinate so as not to overwhelm your system.
+ * active cache operations coordinate so as not to overwhelm your system. When
+ * a request is made for an area of pixels, the operation will block until all
+ * of those pixels have been calculated. Pixels are calculated with a set of
+ * threads. 
  *
  * See also: vips_tilecache().
  *
diff --git a/libvips/conversion/copy.c b/libvips/conversion/copy.c
index d70660b7..f927d849 100644
--- a/libvips/conversion/copy.c
+++ b/libvips/conversion/copy.c
@@ -200,7 +200,8 @@ vips_copy_gen( VipsRegion *or, void *seq, void *a, void *b, gboolean *stop )
 	if( vips_region_prepare( ir, r ) )
 		return( -1 );
 
-	if( copy->swap && swap ) {
+	if( copy->swap && 
+		swap ) {
 		int y;
 
 		for( y = 0; y < r->height; y++ ) {
@@ -239,9 +240,12 @@ static const char *vips_copy_names[] = {
 static int
 vips_copy_build( VipsObject *object )
 {
+	VipsObjectClass *class = VIPS_OBJECT_GET_CLASS( object );
 	VipsConversion *conversion = VIPS_CONVERSION( object );
 	VipsCopy *copy = (VipsCopy *) object;
 
+	guint64 image_size_before;
+	guint64 image_size_after;
 	int i;
 
 	if( VIPS_OBJECT_CLASS( vips_copy_parent_class )->build( object ) )
@@ -254,6 +258,11 @@ vips_copy_build( VipsObject *object )
 		VIPS_DEMAND_STYLE_THINSTRIP, copy->in, NULL ) )
 		return( -1 );
 
+	/* We try to stop the worst crashes by at least ensuring that we don't
+	 * increase the number of pixels which might be addressed.
+	 */
+	image_size_before = VIPS_IMAGE_SIZEOF_IMAGE( conversion->out );
+
 	/* Use props to adjust header fields.
 	 */
 	for( i = 0; i < VIPS_NUMBER( vips_copy_names ); i++ ) {
@@ -291,6 +300,13 @@ vips_copy_build( VipsObject *object )
 		}
 	}
 
+	image_size_after = VIPS_IMAGE_SIZEOF_IMAGE( conversion->out );
+	if( image_size_after > image_size_before ) {
+		vips_error( class->nickname, 
+			"%s", _( "image size too large" ) ); 
+		return( -1 );
+	}
+
 	if( vips_image_generate( conversion->out,
 		vips_start_one, vips_copy_gen, vips_stop_one, 
 		copy->in, copy ) )
@@ -439,7 +455,8 @@ vips_copy_init( VipsCopy *copy )
  * You can optionally set any or all header fields during the copy. Some
  * header fields, such as "xres", the horizontal resolution, are safe to
  * change in any way, others, such as "width" will cause immediate crashes if
- * they are not set carefully. 
+ * they are not set carefully. The operation will block changes which make the
+ * image size grow, see VIPS_IMAGE_SIZEOF_IMAGE(). 
  *
  * Setting @swap to %TRUE will make vips_copy() swap the byte ordering of
  * pixels according to the image's format. 
diff --git a/python/test_conversion.py b/python/test_conversion.py
index a044a7e8..866f51d6 100755
--- a/python/test_conversion.py
+++ b/python/test_conversion.py
@@ -147,5 +147,36 @@ class TestConversion(unittest.TestCase):
 
         self.run_binary(self.all_images, bandrank, fmt = noncomplex_formats)
 
+    def test_cache(self):
+        def cache(x):
+            if isinstance(x, Vips.Image):
+                return x.cache()
+            else:
+                return x
+
+        self.run_unary(self.all_images, cache)
+
+    def test_copy(self):
+        x = self.colour.copy(interpretation = Vips.Interpretation.LAB)
+        self.assertEqual(x.interpretation, Vips.Interpretation.LAB)
+        x = self.colour.copy(xres = 42)
+        self.assertEqual(x.xres, 42)
+        x = self.colour.copy(yres = 42)
+        self.assertEqual(x.yres, 42)
+        x = self.colour.copy(xoffset = 42)
+        self.assertEqual(x.xoffset, 42)
+        x = self.colour.copy(yoffset = 42)
+        self.assertEqual(x.yoffset, 42)
+        x = self.colour.copy(bands = 1)
+        self.assertEqual(x.bands, 1)
+        x = self.colour.copy(format = Vips.BandFormat.USHORT, bands = 1)
+        self.assertEqual(x.format, Vips.BandFormat.USHORT)
+        x = self.colour.copy(coding = Vips.Coding.NONE)
+        self.assertEqual(x.coding, Vips.Coding.NONE)
+        x = self.colour.copy(width = 42)
+        self.assertEqual(x.width, 42)
+        x = self.colour.copy(height = 42)
+        self.assertEqual(x.height, 42)
+
 if __name__ == '__main__':
     unittest.main()