diff --git a/dlls/wined3d/buffer.c b/dlls/wined3d/buffer.c
index 8bb1f635b04..a8fc0d18e60 100644
--- a/dlls/wined3d/buffer.c
+++ b/dlls/wined3d/buffer.c
@@ -275,12 +275,13 @@ static BOOL buffer_process_converted_attribute(struct wined3d_buffer *This,
     attrib_size = attrib->format_desc->component_count * attrib->format_desc->component_size;
     for (i = 0; i < attrib_size; ++i)
     {
-        if (This->conversion_map[data + i] != conversion_type)
+        DWORD_PTR idx = (data + i) % This->stride;
+        if (This->conversion_map[idx] != conversion_type)
         {
-            TRACE("Byte %ld in vertex changed\n", i + data);
-            TRACE("It was type %d, is %d now\n", This->conversion_map[data + i], conversion_type);
+            TRACE("Byte %ld in vertex changed\n", idx);
+            TRACE("It was type %d, is %d now\n", This->conversion_map[idx], conversion_type);
             ret = TRUE;
-            This->conversion_map[data + i] = conversion_type;
+            This->conversion_map[idx] = conversion_type;
         }
     }