diff --git a/src/spline.h b/src/spline.h
index e7bdf15..76ed21e 100644
--- a/src/spline.h
+++ b/src/spline.h
@@ -55,11 +55,11 @@ class band_matrix
     int dim() const;                             // matrix dimension
     int num_upper() const
     {
-        return m_upper.size()-1;
+        return static_cast<int>(m_upper.size()-1);
     }
     int num_lower() const
     {
-        return m_lower.size()-1;
+        return static_cast<int>(m_lower.size()-1);
     }
     // access operator
     double & operator () (int i, int j);            // write
@@ -145,7 +145,7 @@ void band_matrix::resize(int dim, int n_u, int n_l)
 int band_matrix::dim() const
 {
     if(m_upper.size()>0) {
-        return m_upper[0].size();
+        return static_cast<int>(m_upper[0].size());
     } else {
         return 0;
     }
@@ -289,7 +289,7 @@ void spline::set_points(const std::vector<double>& x,
     assert(x.size()>2);
     m_x=x;
     m_y=y;
-    int   n=x.size();
+    int   n= static_cast<int>(x.size());
     // TODO: maybe sort x and y, rather than returning an error
     for(int i=0; i<n-1; i++) {
         assert(m_x[i]<m_x[i+1]);