diff --git a/borgmatic/config/generate.py b/borgmatic/config/generate.py
index 05bbd36..e69d435 100644
--- a/borgmatic/config/generate.py
+++ b/borgmatic/config/generate.py
@@ -230,6 +230,8 @@ def merge_source_configuration_into_destination(destination_config, source_confi
     new
     configuration keys and comments.
     '''
+    if not source_config:
+        return destination_config
     if not destination_config or not isinstance(source_config, collections.abc.Mapping):
         return source_config
 
diff --git a/tests/unit/config/test_generate.py b/tests/unit/config/test_generate.py
index a0a79ba..8e5a07f 100644
--- a/tests/unit/config/test_generate.py
+++ b/tests/unit/config/test_generate.py
@@ -123,3 +123,12 @@ def test_merge_source_configuration_into_destination_inserts_sequence_of_maps():
         'foo': [{'first': 'source1', 'second': 'dest2'}, {'other': 'source2'}],
         'bar': 'dest3',
     }
+
+
+def test_merge_source_configuration_into_destination_without_source_does_nothing():
+    original_destination_config = {'foo': 'dest1', 'bar': 'dest2'}
+    destination_config = dict(original_destination_config)
+
+    module.merge_source_configuration_into_destination(destination_config, None)
+
+    assert destination_config == original_destination_config