diff --git a/src/oemof/solph/_plumbing.py b/src/oemof/solph/_plumbing.py
index 10e9f423d..77c8ffb09 100644
--- a/src/oemof/solph/_plumbing.py
+++ b/src/oemof/solph/_plumbing.py
@@ -59,15 +59,21 @@ def sequence(iterable_or_scalar):
 def valid_sequence(sequence, length: int) -> bool:
     """Checks if an object is a numpy array of at least the given length
     or an 'emulated' sequence object of class _FakeSequence.
-    The latter is set to the required lenght.
+    If unset, the latter is set to the required lenght.
 
     """
     if sequence[0] is None:
         return False
 
     if isinstance(sequence, _FakeSequence):
-        sequence.size = length
-        return True
+        if sequence.size is None:
+            sequence.size = length
+
+        if sequence.size == length:
+            return True
+        else:
+            return False
+
     if isinstance(sequence, np.ndarray):
         if sequence.size == length:
             return True
diff --git a/tests/test_plumbing.py b/tests/test_plumbing.py
index 618f2df7d..f97ff96c1 100644
--- a/tests/test_plumbing.py
+++ b/tests/test_plumbing.py
@@ -81,8 +81,14 @@ def test_valid_sequence():
     assert valid_sequence(fake_sequence, 5)
     assert len(fake_sequence) == 5
 
-    # possible for any length
+    # wil not automatically overwrite size
+    assert not valid_sequence(fake_sequence, 1337)
+    assert len(fake_sequence) == 5
+
+    # manually overwriting length is still possible
+    fake_sequence.size = 1337
     assert valid_sequence(fake_sequence, 1337)
     assert len(fake_sequence) == 1337
 
+    # strings are no valid sequences
     assert not valid_sequence("abc", 3)