diff --git a/src/nested/map/fits/read.rs b/src/nested/map/fits/read.rs
index 0ef9952..fb89aaa 100644
--- a/src/nested/map/fits/read.rs
+++ b/src/nested/map/fits/read.rs
@@ -161,7 +161,7 @@ pub fn from_fits_skymap_internal<R: BufRead>(mut reader: R) -> Result<SkyMapEnum
     _ => unreachable!(),
   }?;
   let n_hash = n_hash(depth);
-  skymap_kws.check_lastpix(n_hash)?;
+  skymap_kws.check_lastpix(n_hash - 1)?;
   // Check whether TForm compatible with N bytes per row
   let n_hash_2 = coltype_1.n_pack() as u64 * n_rows;
   if n_hash != n_hash_2 {
diff --git a/src/nested/map/fits/write.rs b/src/nested/map/fits/write.rs
index 42a7c13..ae45bea 100644
--- a/src/nested/map/fits/write.rs
+++ b/src/nested/map/fits/write.rs
@@ -158,7 +158,7 @@ fn write_skymap_fits_header<R: Write, T: SkyMapValue>(
   it.next().unwrap()[0..20].copy_from_slice(b"EXTNAME = 'xtension'");
   write_uint_mandatory_keyword_record(it.next().unwrap(), b"NSIDE   ", nside as u64);
   it.next().unwrap()[0..30].copy_from_slice(b"FIRSTPIX=                    0");
-  write_uint_mandatory_keyword_record(it.next().unwrap(), b"LASTPIX ", n_cells);
+  write_uint_mandatory_keyword_record(it.next().unwrap(), b"LASTPIX ", n_cells - 1);
   it.next().unwrap()[0..20].copy_from_slice(b"INDXSCHM= 'IMPLICIT'");
   it.next().unwrap()[0..20].copy_from_slice(b"OBJECT  = 'FULLSKY '");
   it.next().unwrap()[0..28].copy_from_slice(b"CREATOR = 'CDS HEALPix Rust'");
diff --git a/src/nested/map/mod.rs b/src/nested/map/mod.rs
index 7aeb63e..07fd0cb 100644
--- a/src/nested/map/mod.rs
+++ b/src/nested/map/mod.rs
@@ -67,5 +67,21 @@ mod tests {
         false,
       )
       .unwrap();
+
+    let path = "test/resources/skymap/hats_test.fits";
+    let skymap = SkyMapEnum::from_fits_file(path).unwrap();
+    skymap
+      .to_png_file::<Mol, _>(
+        (1600, 800),
+        None,
+        None,
+        None,
+        Some(PosConversion::EqMap2GalImg),
+        None,
+        Some(ColorMapFunctionType::LinearLog), //Some(ColorMapFunctionType::LinearSqrt)
+        "test/resources/skymap/hats_tets.png",
+        false,
+      )
+      .unwrap();
   }
 }
diff --git a/src/nested/map/mom/mod.rs b/src/nested/map/mom/mod.rs
index f4556fa..66d30c6 100644
--- a/src/nested/map/mom/mod.rs
+++ b/src/nested/map/mom/mod.rs
@@ -149,6 +149,9 @@ pub trait Mom<'a> {
     }
   }
 
+  /// # Warning
+  /// * assumes that the order of the elements returned by `zuniqs()` is the same as the one returned
+  ///   by `entries()`.
   fn check_is_mom(&'a self) -> Result<(), String> {
     let mut it = self.zuniqs();
     if let Some(mut l) = it.next() {
diff --git a/test/resources/skymap/hats_test.fits b/test/resources/skymap/hats_test.fits
new file mode 100644
index 0000000..0aafa48
Binary files /dev/null and b/test/resources/skymap/hats_test.fits differ
diff --git a/test/resources/skymap/hats_tets.png b/test/resources/skymap/hats_tets.png
new file mode 100644
index 0000000..fa41318
Binary files /dev/null and b/test/resources/skymap/hats_tets.png differ
diff --git a/test/resources/skymap/skymap.fits b/test/resources/skymap/skymap.fits
index a49acc2..999177d 100644
Binary files a/test/resources/skymap/skymap.fits and b/test/resources/skymap/skymap.fits differ