diff --git a/cycler/__init__.py b/cycler/__init__.py
index 1f05670..db476b8 100644
--- a/cycler/__init__.py
+++ b/cycler/__init__.py
@@ -355,6 +355,10 @@ def __iadd__(self, other: Cycler[K, V]) -> Cycler[K, V]:  # type: ignore[misc]
         """
         if not isinstance(other, Cycler):
             raise TypeError("Cannot += with a non-Cycler object")
+        if len(self) != len(other):
+            raise ValueError(
+                f"Can only add equal length cycles, not {len(self)} and {len(other)}"
+            )
         # True shallow copy of self is fine since this is in-place
         old_self = copy.copy(self)
         self._keys = _process_keys(old_self, other)
diff --git a/test_cycler.py b/test_cycler.py
index 637521b..1eccac3 100644
--- a/test_cycler.py
+++ b/test_cycler.py
@@ -65,12 +65,22 @@ def test_prod():
     _cycler_helper(c3 * c1, 45, ['lw', 'c'], target)
 
 
-def test_inplace():
+def test_inplace_add():
     c1 = cycler(c='rgb')
     c2 = cycler(lw=range(3))
     c2 += c1
     _cycler_helper(c2, 3, ['c', 'lw'], [list('rgb'), range(3)])
 
+
+def test_inplace_add_len_mismatch():
+    # miss-matched add lengths
+    c1 = cycler(c='rgb')
+    c3 = cycler(lw=range(15))
+    with pytest.raises(ValueError):
+        c1 += c3
+
+
+def test_inplace_mul():
     c3 = cycler(c='rgb')
     c4 = cycler(lw=range(3))
     c3 *= c4