diff --git a/src/interval/interval.h b/src/interval/interval.h
index 71747f2f3..28f51a138 100644
--- a/src/interval/interval.h
+++ b/src/interval/interval.h
@@ -107,7 +107,7 @@ public:
        \brief Return true iff this contains b.
        That is, every value in b is a value of this.
     */
-    bool contains(interval<T> & b) const;
+    bool contains(interval<T> const & b) const;
 
     bool is_empty() const;
     void set_empty();
diff --git a/src/interval/interval_def.h b/src/interval/interval_def.h
index 6f27b9212..2fc29d661 100644
--- a/src/interval/interval_def.h
+++ b/src/interval/interval_def.h
@@ -104,7 +104,7 @@ bool interval<T>::contains_zero() const {
 }
 
 template<typename T>
-bool interval<T>::contains(interval<T> & b) const {
+bool interval<T>::contains(interval<T> const & b) const {
     if (!m_lower_inf) {
         if (b.m_lower_inf)
             return false;