diff --git a/src/png/scan_lines.rs b/src/png/scan_lines.rs
index 460fd0a0..0b1a6f30 100644
--- a/src/png/scan_lines.rs
+++ b/src/png/scan_lines.rs
@@ -153,6 +153,7 @@ impl Iterator for ScanLineRanges {
         let bits_per_line = pixels_per_line * u32::from(self.bits_per_pixel);
         let bytes_per_line = ((bits_per_line + 7) / 8) as usize;
         let len = bytes_per_line + 1;
+        assert!(self.left >= len);
         self.left -= len;
         Some((len, current_pass))
     }