diff --git a/include/upa/url.h b/include/upa/url.h
index 5202089..6573973 100644
--- a/include/upa/url.h
+++ b/include/upa/url.h
@@ -51,8 +51,8 @@ namespace detail {
     class url_parser;
 
     // url_error what() values
-    extern const char kURLParseError[];
-    extern const char kBaseURLParseError[];
+    extern const char* const kURLParseError;
+    extern const char* const kBaseURLParseError;
 } // namespace detail
 
 /// @brief URL class
diff --git a/src/url.cpp b/src/url.cpp
index 43d4ebb..e3c4d05 100644
--- a/src/url.cpp
+++ b/src/url.cpp
@@ -10,8 +10,8 @@ namespace detail {
 
 // url_error exception what() values
 
-const char kURLParseError[] = "URL parse error";
-const char kBaseURLParseError[] = "Base URL parse error";
+const char* const kURLParseError = "URL parse error";
+const char* const kBaseURLParseError = "Base URL parse error";
 
 // Part start