diff --git a/doc/changes/dev/13582.bugfix.rst b/doc/changes/dev/13582.bugfix.rst
new file mode 100644
index 00000000000..3484e817660
--- /dev/null
+++ b/doc/changes/dev/13582.bugfix.rst
@@ -0,0 +1 @@
+Fix bug where plotting covariance matrices created by :func:`mne.cov.make_ad_hoc_cov` would raise an IndexError, by :newcontrib:`Arnav Kumar` (:gh:`13582`).
diff --git a/doc/changes/names.inc b/doc/changes/names.inc
index 5a5a1e98f89..52490fa9ffc 100644
--- a/doc/changes/names.inc
+++ b/doc/changes/names.inc
@@ -29,6 +29,7 @@
 .. _Antti Rantala: https://github.com/Odingod
 .. _Apoorva Karekal: https://github.com/apoorva6262
 .. _Archit Singhal: https://github.com/architsinghal-mriirs
+.. _Arnav Kumar: https://github.com/Arnav1709
 .. _Arne Pelzer: https://github.com/aplzr
 .. _Ashley Drew: https://github.com/ashdrew
 .. _Asish Panda: https://github.com/kaichogami
diff --git a/mne/viz/misc.py b/mne/viz/misc.py
index a62833d7f81..0b71673f3ad 100644
--- a/mne/viz/misc.py
+++ b/mne/viz/misc.py
@@ -83,7 +83,7 @@ def _index_info_cov(info, cov, exclude):
         for key in _DATA_CH_TYPES_SPLIT
         if len(idx_by_type[key]) > 0
     ]
-    C = cov.data[ch_idx][:, ch_idx]
+    C = cov._get_square()[ch_idx][:, ch_idx]
     return info, C, ch_names, idx_names
 
 
diff --git a/mne/viz/tests/test_misc.py b/mne/viz/tests/test_misc.py
index 7e4a3b2d806..e727b16b495 100644
--- a/mne/viz/tests/test_misc.py
+++ b/mne/viz/tests/test_misc.py
@@ -11,6 +11,7 @@
 
 from mne import (
     SourceEstimate,
+    create_info,
     pick_events,
     read_cov,
     read_dipole,
@@ -19,6 +20,7 @@
     read_source_spaces,
 )
 from mne.chpi import compute_chpi_snr
+from mne.cov import make_ad_hoc_cov
 from mne.datasets import testing
 from mne.filter import create_filter
 from mne.io import read_raw_fif
@@ -136,6 +138,19 @@ def test_plot_cov():
     fig1, fig2 = cov.plot(raw.info)
 
 
+def test_plot_cov_diagonal():
+    """Test plotting of diagonal covariances (e.g., from make_ad_hoc_cov)."""
+    n_channels = 10
+    sfreq = 100
+    info = create_info(
+        [f"EEG{i:03d}" for i in range(n_channels)], sfreq, ch_types="eeg"
+    )
+    cov = make_ad_hoc_cov(info, std={"eeg": 1})
+    # This should not raise an IndexError
+    fig1, fig2 = cov.plot(info)
+    plt.close("all")
+
+
 @testing.requires_testing_data
 def test_plot_bem():
     """Test plotting of BEM contours."""