diff --git a/bindings/CXX11/adios2/cxx11/IO.h b/bindings/CXX11/adios2/cxx11/IO.h
index cce90e6d01..717b6be72b 100644
--- a/bindings/CXX11/adios2/cxx11/IO.h
+++ b/bindings/CXX11/adios2/cxx11/IO.h
@@ -252,7 +252,7 @@ class IO
      * Open an Engine to start heavy-weight input/output operations.
      * @param name unique engine identifier
      * @param mode adios2::Mode::Write, adios2::Mode::Read, or
-     *             adios2::Mode::Append (not yet support)
+     *             adios2::Mode::Append (BP4 only)
      * @return engine object
      */
     Engine Open(const std::string &name, const Mode mode);
@@ -265,7 +265,7 @@ class IO
      * MPI Collective function as it calls MPI_Comm_dup
      * @param name unique engine identifier within IO
      * @param mode adios2::Mode::Write, adios2::Mode::Read, or
-     *             adios2::Mode::Append (not yet support)
+     *             adios2::Mode::Append (BP4 only)
      * @param comm new communicator other than ADIOS object's communicator
      * @return engine object
      */
diff --git a/source/adios2/engine/bp3/BP3Writer.cpp b/source/adios2/engine/bp3/BP3Writer.cpp
index 47b0554b50..9e869c4b64 100644
--- a/source/adios2/engine/bp3/BP3Writer.cpp
+++ b/source/adios2/engine/bp3/BP3Writer.cpp
@@ -204,8 +204,9 @@ void BP3Writer::InitBPBuffer()
 {
     if (m_OpenMode == Mode::Append)
     {
-        throw std::invalid_argument(
-            "ADIOS2: OpenMode Append hasn't been implemented, yet");
+        throw std::invalid_argument("ADIOS2: Mode::Append is only available in "
+                                    "BP4; it is not implemented "
+                                    "for BP3 files.");
         // TODO: Get last pg timestep and update timestep counter in
     }
     else