diff --git a/histogram.cpp b/histogram.cpp
index fb6e26b..8671c2e 100644
--- a/histogram.cpp
+++ b/histogram.cpp
@@ -1,5 +1,6 @@
 #include <vector>
 #include "histogram.h"
+#include "histogram_internal.h"
 void find_minmax(const std:: vector<double>& numbers, double& minimum, double& maximum) {
     minimum = numbers[0];
     maximum = numbers[1];