diff --git a/common/ipprefix.cpp b/common/ipprefix.cpp
index 6fb466101..7d8a3fe7e 100644
--- a/common/ipprefix.cpp
+++ b/common/ipprefix.cpp
@@ -61,7 +61,7 @@ IpPrefix::IpPrefix(const ip_addr_t &ip, int mask) : m_ip(ip), m_mask(mask)
     }
 }
 
-bool IpPrefix::isValid()
+bool IpPrefix::isValid() const
 {
     if (m_mask < 0) return false;
     
diff --git a/common/ipprefix.h b/common/ipprefix.h
index 6a348eb41..5ab7d3936 100644
--- a/common/ipprefix.h
+++ b/common/ipprefix.h
@@ -202,7 +202,7 @@ class IpPrefix
     std::string to_string() const;
 
 private:
-    bool isValid();
+    bool isValid() const;
 
     IpAddress m_ip;
     int m_mask;