--- optipng-0.7.7.old/src/pngxtern/pngxmem.c
+++ optipng-0.7.7/src/pngxtern/pngxmem.c
@@ -35,7 +35,7 @@
    row_size = png_get_rowbytes(png_ptr, info_ptr);
    /* libpng sets row_size to 0 when the width is too large to process. */
    if (row_size == 0 ||
-       (pngx_alloc_size_t)height > (pngx_alloc_size_t)(-1) / sizeof(png_bytep))
+       (pngx_alloc_size_t)height > (pngx_alloc_size_t)(-1U) / sizeof(png_bytep))
       png_error(png_ptr, "Can't handle exceedingly large image dimensions");
    if (row_size < min_row_size)
       row_size = min_row_size;