diff --git a/integer.cpp b/integer.cpp
index 5556aee..7a22541 100644
--- a/integer.cpp
+++ b/integer.cpp
@@ -283,7 +283,7 @@ public:
     return lhs;
   }
 
-  operator unsigned long() {
+  explicit operator unsigned long() {
     unsigned long ret = 0;
     for (std::size_t i = 0; i < std::min(size, sizeof(unsigned long)); i++) {
       ret += (static_cast<unsigned long>(this->bytes[i]) << (i * 8));