diff --git a/src/frame.h b/src/frame.h
index 785b0d06..b61ff10f 100644
--- a/src/frame.h
+++ b/src/frame.h
@@ -219,7 +219,7 @@ class Frame : public FFWrapperPtr<AVFrame>
                                m_raw->extended_buf[plane - AV_NUM_DATA_POINTERS];
         if (buf == nullptr)
             return 0;
-        return buf->size;
+        return size_t(buf->size);
     }
 
     size_t size() const {