diff --git a/Sim/Scan/AlphaScan.cpp b/Sim/Scan/AlphaScan.cpp
index 49b9b7c3e25f7a683266f7fe061be906f77fe461..2a18511204c72110749f2f0b1febd9a21bc0b5a4 100644
--- a/Sim/Scan/AlphaScan.cpp
+++ b/Sim/Scan/AlphaScan.cpp
@@ -27,7 +27,7 @@
 namespace {
 
 std::vector<std::vector<double>>
-extractValues(std::vector<std::vector<ParameterSample>> samples,
+extractValues(const std::vector<std::vector<ParameterSample>>& samples,
               const std::function<double(const ParameterSample&)> extractor)
 {
     std::vector<std::vector<double>> result;