diff --git a/src/catch2/catch_tostring.cpp b/src/catch2/catch_tostring.cpp
index cc686567..20b0a629 100644
--- a/src/catch2/catch_tostring.cpp
+++ b/src/catch2/catch_tostring.cpp
@@ -57,6 +57,12 @@ namespace Detail {
         }
     } // end unnamed namespace
 
+    std::size_t catch_strnlen( const char* str, std::size_t n ) {
+        auto ret = std::char_traits<char>::find( str, n, '\0' );
+        if ( ret != nullptr ) { return static_cast<std::size_t>( ret - str ); }
+        return n;
+    }
+
     std::string convertIntoString(StringRef string, bool escapeInvisibles) {
         std::string ret;
         // This is enough for the "don't escape invisibles" case, and a good
diff --git a/src/catch2/catch_tostring.hpp b/src/catch2/catch_tostring.hpp
index 41f89413..aca1f7e0 100644
--- a/src/catch2/catch_tostring.hpp
+++ b/src/catch2/catch_tostring.hpp
@@ -40,13 +40,7 @@ namespace Catch {
 
     namespace Detail {
 
-        inline std::size_t catch_strnlen(const char *str, std::size_t n) {
-            auto ret = std::char_traits<char>::find(str, n, '\0');
-            if (ret != nullptr) {
-                return static_cast<std::size_t>(ret - str);
-            }
-            return n;
-        }
+        std::size_t catch_strnlen(const char *str, std::size_t n);
 
         constexpr StringRef unprintableString = "{?}"_sr;