diff --git a/tests/cpp_tests/image_io_test.cpp b/tests/cpp_tests/image_io_test.cpp
index aae80ede0..2767bc2e0 100644
--- a/tests/cpp_tests/image_io_test.cpp
+++ b/tests/cpp_tests/image_io_test.cpp
@@ -23,7 +23,7 @@ int main(int argc, char** argv)
     {
         mapnik::image_data_32 im(256,256);
         unsigned char* bytes = im.getBytes();
-        mapnik::image_data_32 * im_ptr = new mapnik::image_data_32(256,256,(unsigned int *)bytes);
+        mapnik::image_data_32 * im_ptr = new mapnik::image_data_32(im.width(),im.height(),static_cast<mapnik::image_data_32::pixel_type *>(bytes));
         unsigned char* same_bytes = im_ptr->getBytes();
         BOOST_TEST(bytes == same_bytes);
         delete im_ptr;