diff --git a/msl_verify.hpp b/msl_verify.hpp
index 59f4017..d3015d9 100644
--- a/msl_verify.hpp
+++ b/msl_verify.hpp
@@ -25,7 +25,7 @@
     /** @brief Constructs MinimumShipLevel.
      *  @param[in] minShipLevel - Minimum Ship Level string
      */
-    MinimumShipLevel(const std::string& minShipLevel) :
+    explicit MinimumShipLevel(const std::string& minShipLevel) :
         minShipLevel(minShipLevel){};
 
     /** @brief Verify if the current PNOR version meets the min ship level