diff --git a/src/borg/chunkers/reader.pyx b/src/borg/chunkers/reader.pyx
index d33917a3a..115281d42 100644
--- a/src/borg/chunkers/reader.pyx
+++ b/src/borg/chunkers/reader.pyx
@@ -138,7 +138,10 @@ class FileFMAPReader:
             try:
                 fmap = list(sparsemap(self.fd, self.fh))
             except (OSError, ValueError) as err:
-                # seeking did not work
+                # Building a sparse map failed:
+                # - OSError: low-level lseek with SEEK_HOLE/SEEK_DATA not supported by FS/OS.
+                # - ValueError: high-level file objects (e.g. io.BytesIO or some fd wrappers)
+                #   don't accept SEEK_HOLE/SEEK_DATA as a valid "whence" and raise ValueError.
                 pass
 
         if fmap is None:
diff --git a/src/borg/testsuite/chunkers/fixed_test.py b/src/borg/testsuite/chunkers/fixed_test.py
index 8fac894f1..80782d4d0 100644
--- a/src/borg/testsuite/chunkers/fixed_test.py
+++ b/src/borg/testsuite/chunkers/fixed_test.py
@@ -34,7 +34,7 @@ from ...constants import *  # NOQA
 )
 def test_chunkify_sparse(tmpdir, fname, sparse_map, header_size, sparse):
     def get_chunks(fname, sparse, header_size):
-        chunker = ChunkerFixed(4096, header_size=header_size, sparse=sparse)
+        chunker = ChunkerFixed(BS, header_size=header_size, sparse=sparse)
         with open(fname, "rb") as fd:
             return cf(chunker.chunkify(fd))