diff --git a/source/marathon/defl.rs b/source/marathon/defl.rs
index 72b9404..23f60d2 100644
--- a/source/marathon/defl.rs
+++ b/source/marathon/defl.rs
@@ -18,10 +18,18 @@ pub fn load_zlib_header(b: &[u8]) -> ResultS<usize>
    }
 
    let cm    = cmf & CM;
-   let cinfo = cmf & CINFO >> 4;
+   let cinfo = cmf & CINFO;
 
-   if cm != 8 || fcheck % 31 != 0 || cinfo > 7 {
-      bail!("not zlib format");
+   if cm != 8 {
+      bail!("unknown compression method");
+   }
+
+   if cinfo > 7 << 4 {
+      bail!("lz77 window size logarithm is invalid");
+   }
+
+   if fcheck % 31 != 0 {
+      bail!("invalid fcheck");
    }
 
    if flg & FDICT != 0 {