diff --git a/pysteps/tests/test_verification_detcontscores_merge.py b/pysteps/tests/test_verification_detcontscores_merge.py
new file mode 100644
index 000000000..58ca9db87
--- /dev/null
+++ b/pysteps/tests/test_verification_detcontscores_merge.py
@@ -0,0 +1,45 @@
+"""Merging reusable verification summaries must preserve their inputs."""
+
+from copy import deepcopy
+
+import numpy as np
+import pytest
+
+from pysteps.verification import detcontscores
+
+
+@pytest.mark.parametrize("axis", [None, 0, (1, 2), -1])
+def test_merge_preserves_inputs(axis):
+    rng = np.random.default_rng(42)
+    pred = rng.uniform(0, 10, size=(2, 2, 3, 4))
+    obs = rng.uniform(0, 10, size=pred.shape)
+    summaries = []
+    expected = detcontscores.det_cont_fct_init(axis=axis)
+    for p, o in zip(pred, obs):
+        summary = detcontscores.det_cont_fct_init(axis=axis)
+        detcontscores.det_cont_fct_accum(summary, p, o)
+        detcontscores.det_cont_fct_accum(expected, p, o)
+        summaries.append(summary)
+
+    before = deepcopy(summaries)
+    first, second = summaries
+    merged = detcontscores.det_cont_fct_merge(first, second)
+    for summary, original in zip(summaries, before):
+        for key in original:
+            np.testing.assert_equal(summary[key], original[key])
+
+    # The same partial summaries may be reused for another aggregate.
+    repeated = detcontscores.det_cont_fct_merge(first, second)
+    reverse = detcontscores.det_cont_fct_merge(second, first)
+    for key, value in expected.items():
+        if isinstance(value, np.ndarray):
+            for result in (merged, repeated, reverse):
+                np.testing.assert_allclose(result[key], value)
+                assert not np.shares_memory(result[key], summaries[0][key])
+                assert not np.shares_memory(result[key], summaries[1][key])
+
+    # Updating the merged object must also leave the partial summaries alone.
+    detcontscores.det_cont_fct_accum(merged, pred[0], obs[0])
+    for summary, original in zip(summaries, before):
+        for key in original:
+            np.testing.assert_equal(summary[key], original[key])
diff --git a/pysteps/verification/detcontscores.py b/pysteps/verification/detcontscores.py
index 25d989016..f483f1313 100644
--- a/pysteps/verification/detcontscores.py
+++ b/pysteps/verification/detcontscores.py
@@ -16,6 +16,7 @@
 """
 
 import collections
+import copy
 import numpy as np
 from scipy.stats import spearmanr
 
@@ -433,7 +434,7 @@ def det_cont_fct_merge(err_1, err_2):
         raise ValueError("cannot merge: no data found")
 
     # merge the two verification error objects
-    err = err_1.copy()
+    err = copy.deepcopy(err_1)
 
     # update variances
     _parallel_var(