diff --git a/lethe.py b/lethe.py
index 71e700c..5915a64 100755
--- a/lethe.py
+++ b/lethe.py
@@ -109,6 +109,12 @@ def find_merge_base(commits: List[str], cwd: str=None) -> str:
     """
     Find the "best common ancestor" commit.
     """
+    if len(commits) == 0:
+        raise Exception('Called find_merge_base with no commits!')
+
+    if len(commits) == 1:
+        return commits[0]
+
     base = _run(['git', 'merge-base', *commits], cwd=cwd)
     return base