diff --git a/src/utils.cpp b/src/utils.cpp
index 9d32f3b..3b611d7 100644
--- a/src/utils.cpp
+++ b/src/utils.cpp
@@ -113,6 +113,7 @@
             throw std::runtime_error("Error reading mapper response");
         }
         std::vector<std::string> ret;
+        ret.reserve(mapperResponse.size());
         for (const auto& i : mapperResponse)
         {
             ret.emplace_back(i.first);