diff --git a/src/pio_mpi_fw_at_reblock.c b/src/pio_mpi_fw_at_reblock.c
index 3b2f7871e8f4f72093dc46cee74622e14974795b..3e41e5d4a4fe9b5d39c0dffa2d78a2c64e3151ba 100644
--- a/src/pio_mpi_fw_at_reblock.c
+++ b/src/pio_mpi_fw_at_reblock.c
@@ -184,7 +184,7 @@ initAFiledataFileWriteAtReblock(struct fileMPIFWAR *of, const char *filename,
   of->name = Malloc(nameSize);
   memcpy(of->name, filename, nameSize);
   {
-    void *ptr;
+    void *ptr = NULL;
     int err = posix_memalign(&ptr, bufBlockAlign, bufSize);
     if (!err)
       of->blockBuf = ptr;