diff --git a/xs/src/boost/nowide/cenv.hpp b/xs/src/boost/nowide/cenv.hpp
index 5f6d084af..5b41b8e8d 100755
--- a/xs/src/boost/nowide/cenv.hpp
+++ b/xs/src/boost/nowide/cenv.hpp
@@ -99,9 +99,11 @@ namespace nowide {
     ///
     inline int putenv(char *string)
     {
+        if (string == nullptr) return -1;
         char const *key = string;
         char const *key_end = string;
-        while(*key_end!='=' && key_end!='\0')
+        
+        while(*key_end!='=' && *key_end!='\0')
             key_end++;
         if(*key_end == '\0')
             return -1;