diff --git a/core/fxcodec/codec/fx_codec_fax.cpp b/core/fxcodec/codec/fx_codec_fax.cpp
index 4a70b53..0a6ba3a 100644
--- a/core/fxcodec/codec/fx_codec_fax.cpp
+++ b/core/fxcodec/codec/fx_codec_fax.cpp
@@ -544,8 +544,10 @@
       m_bitpos = bitpos1;
   }
   if (m_bBlack) {
-    for (uint32_t i = 0; i < m_Pitch; ++i)
-      m_ScanlineBuf[i] = ~m_ScanlineBuf[i];
+    ASSERT(m_Pitch == m_ScanlineBuf.size());
+    uint8_t* data = m_ScanlineBuf.data();
+    for (size_t i = 0; i < m_ScanlineBuf.size(); ++i)
+      data[i] = ~data[i];
   }
   return m_ScanlineBuf.data();
 }