diff --git a/capa/features/extractors/null.py b/capa/features/extractors/null.py
index 50bd85114..f6797aa96 100644
--- a/capa/features/extractors/null.py
+++ b/capa/features/extractors/null.py
@@ -59,6 +59,9 @@ class NullStaticFeatureExtractor(StaticFeatureExtractor):
     def get_base_address(self):
         return self.base_address
 
+    def get_sample_hashes(self) -> SampleHashes:
+        return self.sample_hashes
+
     def extract_global_features(self):
         for feature in self.global_features:
             yield feature, NO_ADDRESS
@@ -121,6 +124,9 @@ def extract_global_features(self):
         for feature in self.global_features:
             yield feature, NO_ADDRESS
 
+    def get_sample_hashes(self) -> SampleHashes:
+        return self.sample_hashes
+
     def extract_file_features(self):
         for address, feature in self.file_features:
             yield feature, address
diff --git a/capa/rules/__init__.py b/capa/rules/__init__.py
index 04ea11bd2..35f2a0907 100644
--- a/capa/rules/__init__.py
+++ b/capa/rules/__init__.py
@@ -86,6 +86,10 @@ class Scope(str, Enum):
     # not used to validate rules.
     GLOBAL = "global"
 
+    @classmethod
+    def to_yaml(cls, representer, node):
+        return representer.represent_str(f"{node.value}")
+
 
 # these literals are used to check if the flavor
 # of a rule is correct.
@@ -979,6 +983,7 @@ def _get_ruamel_yaml_parser():
 
         # we use the ruamel.yaml parser because it supports roundtripping of documents with comments.
         y = ruamel.yaml.YAML(typ="rt")
+        y.register_class(Scope)
 
         # use block mode, not inline json-like mode
         y.default_flow_style = False
diff --git a/tests/test_result_document.py b/tests/test_result_document.py
index 0311a1d69..10f022d94 100644
--- a/tests/test_result_document.py
+++ b/tests/test_result_document.py
@@ -263,7 +263,6 @@ def assert_round_trip(rd: rdoc.ResultDocument):
         pytest.param("a076114_rd"),
         pytest.param("pma0101_rd"),
         pytest.param("dotnet_1c444e_rd"),
-        pytest.param(""),
     ],
 )
 def test_round_trip(request, rd_file):