diff --git a/opm/core/GridManager.cpp b/opm/core/GridManager.cpp
index 2f3db42a..f84d1c55 100644
--- a/opm/core/GridManager.cpp
+++ b/opm/core/GridManager.cpp
@@ -153,7 +153,7 @@ namespace Opm
 
     namespace
     {
-        std::vector<double> coordsFromDeltas(const std::vector<double> deltas)
+        std::vector<double> coordsFromDeltas(const std::vector<double>& deltas)
         {
             std::vector<double> coords(deltas.size() + 1);
             coords[0] = 0.0;