diff --git a/src/index/DeltaTriples.cpp b/src/index/DeltaTriples.cpp
index de0764a0d6..23776adfa9 100644
--- a/src/index/DeltaTriples.cpp
+++ b/src/index/DeltaTriples.cpp
@@ -200,10 +200,10 @@ void to_json(nlohmann::json& j, const DeltaTriplesCount& count) {
 }
 
 // ____________________________________________________________________________
-DeltaTriplesCount DeltaTriplesCount::operator-(
-    const DeltaTriplesCount& other) const {
-  return {triplesInserted_ - other.triplesInserted_,
-          triplesDeleted_ - other.triplesDeleted_};
+DeltaTriplesCount operator-(const DeltaTriplesCount& lhs,
+                            const DeltaTriplesCount& rhs) {
+  return {lhs.triplesInserted_ - rhs.triplesInserted_,
+          lhs.triplesDeleted_ - rhs.triplesDeleted_};
 }
 
 // ____________________________________________________________________________
diff --git a/src/index/DeltaTriples.h b/src/index/DeltaTriples.h
index 91dac7f237..4c63cef01c 100644
--- a/src/index/DeltaTriples.h
+++ b/src/index/DeltaTriples.h
@@ -47,7 +47,8 @@ struct DeltaTriplesCount {
   /// library to allow for implicit conversion.
   friend void to_json(nlohmann::json& j, const DeltaTriplesCount& count);
 
-  DeltaTriplesCount operator-(const DeltaTriplesCount& other) const;
+  friend DeltaTriplesCount operator-(const DeltaTriplesCount& lhs,
+                                     const DeltaTriplesCount& rhs);
 };
 
 // A class for maintaining triples that are inserted or deleted after index