diff --git a/doc/changes/devel/xxxxx.bugfix.rst b/doc/changes/devel/xxxxx.bugfix.rst
new file mode 100644
index 00000000000..cd6c9623cc3
--- /dev/null
+++ b/doc/changes/devel/xxxxx.bugfix.rst
@@ -0,0 +1 @@
+Fix ``mne.io.edf.edf._parse_prefilter_string`` to add prefilter information as ``np.nan`` when the prefilter information does not exist, by `Michiru Kaneda`_
diff --git a/mne/io/edf/edf.py b/mne/io/edf/edf.py
index a8daf37c2cd..68a661242dc 100644
--- a/mne/io/edf/edf.py
+++ b/mne/io/edf/edf.py
@@ -763,7 +763,7 @@ def _extract_filter_value(s, prefix, suffix):
         start = s.find(prefix) + len(prefix)
         end = s.find(suffix, start)
         return s[start:end].strip()
-    return None
+    return np.nan
 
 
 def _parse_prefilter_string(prefiltering):
diff --git a/mne/io/edf/tests/test_edf.py b/mne/io/edf/tests/test_edf.py
index a324436a049..9ce0902b9ff 100644
--- a/mne/io/edf/tests/test_edf.py
+++ b/mne/io/edf/tests/test_edf.py
@@ -644,8 +644,8 @@ def test_edf_prefilter_parse():
 
     prefilter_unfiltered_ch = prefilter_normal_multi_ch + [""]
     highpass, lowpass = _parse_prefilter_string(prefilter_unfiltered_ch)
-    assert_array_equal(highpass, ["1"] * 10)
-    assert_array_equal(lowpass, ["30"] * 10)
+    assert_array_equal(highpass, ["1"] * 10 + [np.nan])
+    assert_array_equal(lowpass, ["30"] * 10 + [np.nan])
 
     prefilter_edf_specs_doc = ["HP:0.1Hz LP:75Hz N:50Hz"]
     highpass, lowpass = _parse_prefilter_string(prefilter_edf_specs_doc)
@@ -654,8 +654,8 @@ def test_edf_prefilter_parse():
 
     prefilter_edf_specs_doc = ["", "HP:0.1Hz LP:75Hz N:50Hz", ""]
     highpass, lowpass = _parse_prefilter_string(prefilter_edf_specs_doc)
-    assert_array_equal(highpass, [None, "0.1", None])
-    assert_array_equal(lowpass, [None, "75", None])
+    assert_array_equal(highpass, [np.nan, "0.1", np.nan])
+    assert_array_equal(lowpass, [np.nan, "75", np.nan])
 
 
 @testing.requires_testing_data