diff --git a/histogram_internal.h b/histogram_internal.h
index a73f166..56f34af 100644
--- a/histogram_internal.h
+++ b/histogram_internal.h
@@ -3,6 +3,6 @@
 #include <vector>
 #include <iostream>
 bool
-find_minmax(const std::vector<double> numbers, double& min, double& max);
+find_minmax(const std::vector<double>& numbers, double& min, double& max);
 
-#endif
\ No newline at end of file
+#endif