diff --git a/src/lib/base/String.cpp b/src/lib/base/String.cpp
index 65afd3745..86031d741 100644
--- a/src/lib/base/String.cpp
+++ b/src/lib/base/String.cpp
@@ -145,7 +145,7 @@ std::string sizeTypeToString(size_t n)
   return ss.str();
 }
 
-size_t stringToSizeType(std::string string)
+size_t stringToSizeType(const std::string &string)
 {
   std::istringstream iss(string);
   size_t value;
diff --git a/src/lib/base/String.h b/src/lib/base/String.h
index 89a40d0ee..9c8df53d1 100644
--- a/src/lib/base/String.h
+++ b/src/lib/base/String.h
@@ -52,7 +52,7 @@ std::string sizeTypeToString(size_t n);
 /*!
 Convert an a \c string to an size type
 */
-size_t stringToSizeType(std::string string);
+size_t stringToSizeType(const std::string &string);
 
 //! Case-insensitive comparisons
 /*!