diff --git a/histogram.cpp b/histogram.cpp
index f62a31d..319e9d5 100644
--- a/histogram.cpp
+++ b/histogram.cpp
@@ -20,12 +20,12 @@ void find_minmax(const std::vector <double> &numbers, double &min, double &max,
     }
 }
 
-std::vector <int> make_histogram( std::vector<double> &numbers, size_t &bin_count) {
+std::vector <std::size_t> make_histogram( const std::vector<double> &numbers, std::size_t bin_count) {
     double min, max;
     bool res = false;
     find_minmax(numbers, min, max, res);
     double bin_size = ( max - min ) / bin_count;
-    std::vector<int> bins ( bin_count );
+    std::vector<std::size_t> bins ( bin_count );
     for (std::size_t i=0; i <  numbers.size(); i++ ){
         bool found = false;
         for (std::size_t j = 0; ( j < bin_count - 1 ) && !found; j++ ){
diff --git a/histogram.h b/histogram.h
index 69a04c7..7eddad5 100644
--- a/histogram.h
+++ b/histogram.h
@@ -3,7 +3,6 @@
 
 #include <vector>
 
-std::vector<int>
-make_histogram(const std::vector<double> &numbers, std::size_t &bin_count);
+std::vector <std::size_t> make_histogram(const std::vector<double> &numbers, std::size_t bin_count);
 
 #endif // HISTOGRAM_H_INCLUDED
diff --git a/svg.cpp b/svg.cpp
index abc0a77..6c8689b 100644
--- a/svg.cpp
+++ b/svg.cpp
@@ -27,7 +27,7 @@ void svg_rect(double x, double y, double width, double height){
 }
 
 void
-show_histogram_svg(const std::vector <int> bins) {
+show_histogram_svg(const std::vector <std::size_t> &bins) {
 
     const auto IMAGE_WIDTH = 400;
     const auto IMAGE_HEIGHT = 300;
diff --git a/svg.h b/svg.h
index a09d6d2..c159f7d 100644
--- a/svg.h
+++ b/svg.h
@@ -3,7 +3,6 @@
 
 #include <vector>
 
-void
-show_histogram_svg(const std::vector<int> &bins);
+void show_histogram_svg(const std::vector<std::size_t> &bins);
 
 #endif // SVG_H_INCLUDED