diff --git a/src/codemodder/result.py b/src/codemodder/result.py
index b268c7d2..0c1743ed 100644
--- a/src/codemodder/result.py
+++ b/src/codemodder/result.py
@@ -1,5 +1,6 @@
 from dataclasses import dataclass
 from pathlib import Path
+from typing import Any
 
 from .utils.abc_dataclass import ABCDataclass
 
@@ -49,4 +50,16 @@ def all_rule_ids(self) -> list[str]:
         return list(self.keys())
 
     def __or__(self, other):
-        return ResultSet(super().__or__(other))
+        result = ResultSet(super().__or__(other))
+        for k in self.keys() | other.keys():
+            result[k] = list_dict_or(self[k], other[k])
+        return result
+
+
+def list_dict_or(
+    dictionary: dict[Any, list[Any]], other: dict[Any, list[Any]]
+) -> dict[Path, list[Any]]:
+    result_dict = other | dictionary
+    for k in other.keys() | dictionary.keys():
+        result_dict[k] = dictionary[k] + other[k]
+    return result_dict
diff --git a/tests/test_results.py b/tests/test_results.py
new file mode 100644
index 00000000..5f34bfd6
--- /dev/null
+++ b/tests/test_results.py
@@ -0,0 +1,55 @@
+import json
+from pathlib import Path
+from codemodder.sonar_results import SonarResultSet
+
+
+class TestResults:
+    def test_or(self, tmpdir):
+        issues1 = {
+            "issues": [
+                {
+                    "rule": "rule",
+                    "component": "code.py",
+                    "textRange": {
+                        "startLine": 2,
+                        "endLine": 2,
+                        "startOffset": 2,
+                        "endOffset": 2,
+                    },
+                }
+            ]
+        }
+        issues2 = {
+            "issues": [
+                {
+                    "rule": "rule",
+                    "component": "code.py",
+                    "textRange": {
+                        "startLine": 1,
+                        "endLine": 1,
+                        "startOffset": 1,
+                        "endOffset": 1,
+                    },
+                }
+            ]
+        }
+        sonar_json1 = Path(tmpdir) / "sonar1.json"
+        sonar_json1.write_text(json.dumps(issues1))
+        sonar_json2 = Path(tmpdir) / "sonar2.json"
+        sonar_json2.write_text(json.dumps(issues2))
+
+        result1 = SonarResultSet.from_json(sonar_json1)
+        result2 = SonarResultSet.from_json(sonar_json2)
+
+        combined = result1 | result2
+        assert len(combined["rule"][Path("code.py")]) == 2
+        assert result2["rule"][Path("code.py")][0] in combined["rule"][Path("code.py")]
+        assert result1["rule"][Path("code.py")][0] in combined["rule"][Path("code.py")]
+
+    def test_sonar_robustness(self, tmpdir):
+        sonar_json = Path(tmpdir) / "sonar1.json"
+        # not a valid json
+        sonar_json.touch()
+        result = SonarResultSet.from_json(sonar_json)
+        # did not crash and returned an empty ResultSet
+        assert not result