diff --git a/modules/beast_core/text/beast_String.cpp b/modules/beast_core/text/beast_String.cpp
index 4fad9e955..042c05793 100644
--- a/modules/beast_core/text/beast_String.cpp
+++ b/modules/beast_core/text/beast_String.cpp
@@ -131,7 +131,8 @@ public:
         if (start.getAddress() == nullptr || start.isEmpty())
             return getEmpty();
 
-        const size_t numBytes = (size_t) (end.getAddress() - start.getAddress());
+        const size_t numBytes = (size_t)(  reinterpret_cast<const char*> (end.getAddress())
+                                         - reinterpret_cast<const char*> (start.getAddress()));
         const CharPointerType dest (createUninitialisedBytes (numBytes + sizeof (CharType)));
         memcpy (dest.getAddress(), start, numBytes);
         dest.getAddress()[numBytes / sizeof (CharType)] = 0;