diff --git a/ompi/mca/io/romio321/romio/mpi-io/mpioimpl.h b/ompi/mca/io/romio321/romio/mpi-io/mpioimpl.h
index 7c5f0eb3947..85f7fa70f47 100644
--- a/ompi/mca/io/romio321/romio/mpi-io/mpioimpl.h
+++ b/ompi/mca/io/romio321/romio/mpi-io/mpioimpl.h
@@ -36,7 +36,12 @@
 #define ROMIO_THREAD_CS_ENTER()
 #define ROMIO_THREAD_CS_EXIT()
 #define ROMIO_THREAD_CS_YIELD()
-#define MPIO_DATATYPE_ISCOMMITTED(dtype_, err_) do {} while (0)
+/* The MPI_DATATYPE_ISCOMMITTED macro now always sets err_=0.
+   This is an optimistic approach for Open MPI, but it is likely other
+   upper layers already checked the datatype was committed.
+   Not setting err_ is incorrect since it can lead to use of
+   uninitialized variable.*/
+#define MPIO_DATATYPE_ISCOMMITTED(dtype_, err_) do { err_ = 0; } while (0)
 #ifdef HAVE_WINDOWS_H
 #define MPIU_UNREFERENCED_ARG(a) a
 #else