diff --git a/pb_encode.c b/pb_encode.c
index f5f1676..2ad9925 100644
--- a/pb_encode.c
+++ b/pb_encode.c
@@ -65,7 +65,11 @@
 {
     pb_ostream_t stream;
 #ifdef PB_BUFFER_ONLY
-    stream.callback = (void*)1; /* Just a marker value */
+    /* In PB_BUFFER_ONLY configuration the callback pointer is just int*.
+     * NULL pointer marks a sizing field, so put a non-NULL value to mark a buffer stream.
+     */
+    static const int marker;
+    stream.callback = &marker;
 #else
     stream.callback = &buf_write;
 #endif
diff --git a/pb_encode.h b/pb_encode.h
index 9cff22a..8913683 100644
--- a/pb_encode.h
+++ b/pb_encode.h
@@ -33,7 +33,7 @@
      * Also, NULL pointer marks a 'sizing stream' that does not
      * write anything.
      */
-    int *callback;
+    const int *callback;
 #else
     bool (*callback)(pb_ostream_t *stream, const pb_byte_t *buf, size_t count);
 #endif