diff --git a/src/image.cpp b/src/image.cpp
index 818af4e7..689f702a 100644
--- a/src/image.cpp
+++ b/src/image.cpp
@@ -1005,12 +1005,18 @@ namespace Exiv2 {
         size_t need = std::strlen(format);                 // initial guess
         int    rc   = -1;
 
+        // vsnprintf writes at most size (2nd parameter) bytes (including \0)
+        //           returns the number of bytes required for the formatted string excluding \0
+        // the following loop goes through:
+        // one iteration (if 'need' was large enough for the for formatted string)
+        // or two iterations (after the first call to vsnprintf we know the required length)
         do {
             buffer.resize(need + 1);
             va_list  args;                                 // variable arg list
             va_start(args, format);                        // args start after format
             rc = vsnprintf(&buffer[0], buffer.size(), format, args);
             va_end(args);                                  // free the args
+            assert(rc >= 0);                               // rc < 0 => we have made an error in the format string
             if ( rc > 0 )
                 need = static_cast<size_t>(rc);
         } while ( buffer.size() <= need );