diff --git a/include/metalldata/metall_graph.hpp b/include/metalldata/metall_graph.hpp
index 09e586a..7d94e5a 100644
--- a/include/metalldata/metall_graph.hpp
+++ b/include/metalldata/metall_graph.hpp
@@ -490,9 +490,9 @@ class metall_graph {
   //   std::optional<std::string> v_included;
   // };
 
-  return_code nhops(series_name out_node_series, size_t nhops,
-                    std::vector<std::string> sources,
-                    const where_clause&      where = where_clause());
+  return_code nhops(const series_name& out_node_series, size_t nhops,
+                    const std::vector<std::string>& sources,
+                    const where_clause&             where = where_clause());
 
   // TODO: also allow val a function
   return_code assign(series_name series_name, const data_types& val,
diff --git a/src/libmetalldata/metall_graph.cpp b/src/libmetalldata/metall_graph.cpp
index 9c19441..d553918 100644
--- a/src/libmetalldata/metall_graph.cpp
+++ b/src/libmetalldata/metall_graph.cpp
@@ -551,10 +551,9 @@ metall_graph::return_code metall_graph::degrees2(
   return to_return;
 }
 
-metall_graph::return_code metall_graph::nhops(series_name              out_name,
-                                              size_t                   nhops,
-                                              std::vector<std::string> sources,
-                                              const where_clause&      where) {
+metall_graph::return_code metall_graph::nhops(
+  const series_name& out_name, size_t nhops,
+  const std::vector<std::string>& sources, const where_clause& where) {
   return_code to_return;
 
   if (!out_name.is_node_series()) {