diff --git a/tests/test_plumbing.py b/tests/test_plumbing.py
index ecf80ef70..b51187be8 100644
--- a/tests/test_plumbing.py
+++ b/tests/test_plumbing.py
@@ -6,34 +6,60 @@
 
 SPDX-License-Identifier: MIT
 """
+import numpy as np
 import pytest
 
+from oemof.solph._plumbing import _FakeSequence
 from oemof.solph._plumbing import sequence
 
 
-def test_sequence():
-    seq0 = sequence(0)
-    assert seq0[0] == 0
+def test_fake_sequence():
+    seq0 = _FakeSequence(42)
+    assert seq0[0] == 42
     assert seq0.size is None
 
-    assert seq0[10] == 0
+    assert seq0[10] == 42
     assert seq0.size is None
 
-    assert seq0.max() == 0
+    assert seq0.max() == 42
+    assert seq0.min() == 42
+    assert seq0.value == 42
+    assert seq0.sum() == np.inf
+
+    assert str(seq0) == "[42, 42, ..., 42]"
+
+    with pytest.raises(TypeError):
+        seq0.to_numpy()
+    assert (seq0.to_numpy(length=5) == np.array(5 * [42])).all()
+
+    with pytest.raises(TypeError):
+        len(seq0)
+
+    seq0.size = 2
+    assert seq0.size == 2
+    assert len(seq0) == 2
 
-    seq10 = sequence(10)
-    assert seq10.max() == 10
+    assert seq0.max() == 42
+    assert seq0.min() == 42
+    assert seq0.value == 42
+    assert seq0.sum() == 84
 
-    assert seq10[0] == 10
-    assert seq10.size is None
+    assert str(seq0) == "[42, 42]"
 
-    seq10.size = 10
-    assert seq10.size == 10
+    assert (seq0.to_numpy() == np.array(2 * [42])).all()
+    assert (seq0.to_numpy(length=5) == np.array(5 * [42])).all()
+
+
+def test_sequence():
+    seq0 = sequence(0)
+    assert isinstance(seq0, _FakeSequence)
+    assert seq0.value == 0
+    assert seq0.size is None
 
-    seq12 = sequence([1, 3])
-    assert max(seq12) == 3
-    assert seq12[0] == 1
-    assert seq12[1] == 3
+    seq13 = sequence([1, 3])
+    assert isinstance(seq13, np.ndarray)
+    assert (seq13 == np.array([1, 3])).all()
 
-    with pytest.raises(IndexError):
-        _ = seq12[2]
+    seq_ab = sequence("ab")
+    assert isinstance(seq_ab, str)
+    assert seq_ab == "ab"