diff --git a/cmaes/_warm_start.py b/cmaes/_warm_start.py
index fb8ab56..a915bdf 100644
--- a/cmaes/_warm_start.py
+++ b/cmaes/_warm_start.py
@@ -37,7 +37,7 @@ def get_warm_start_mgd(
     source_solutions = sorted(source_solutions, key=lambda t: t[1])
     gamma_n = math.floor(len(source_solutions) * gamma)
     assert gamma_n >= 1, "One or more solutions must be selected from a source task"
-    dim = len(source_solutions[0])
+    dim = len(source_solutions[0][0])
     top_gamma_solutions = np.empty(
         shape=(
             gamma_n,
diff --git a/tests/test_warm_start.py b/tests/test_warm_start.py
new file mode 100644
index 0000000..3f747a3
--- /dev/null
+++ b/tests/test_warm_start.py
@@ -0,0 +1,12 @@
+import numpy as np
+from unittest import TestCase
+from cmaes import CMA, get_warm_start_mgd
+
+
+class TestWarmStartCMA(TestCase):
+    def test_dimension(self):
+        optimizer = CMA(mean=np.zeros(10), sigma=1.3)
+        source_solutions = [(optimizer.ask(), 0.0) for _ in range(100)]
+        ws_mean, ws_sigma, ws_cov = get_warm_start_mgd(source_solutions)
+
+        self.assertEqual(ws_mean.size, 10)