diff --git a/src/driver/tpax_driver_ctx.c b/src/driver/tpax_driver_ctx.c
index 6ed71ab..6497126 100644
--- a/src/driver/tpax_driver_ctx.c
+++ b/src/driver/tpax_driver_ctx.c
@@ -418,7 +418,7 @@ int tpax_lib_get_driver_ctx(
 					for (; *ch; ch++)
 						if ((*ch < '0') || (*ch > '9'))
 							return tpax_driver_usage_block_size(
-								fdctx->fdout,
+								fdctx->fderr,
 								program,entry->arg,
 								optv,meta);
 
@@ -426,7 +426,7 @@ int tpax_lib_get_driver_ctx(
 
 					if ((cctx.blksize < 512) || (cctx.blksize > 32256))
 						return tpax_driver_usage_block_size_range(
-							fdctx->fdout,
+							fdctx->fderr,
 							program,entry->arg,
 							optv,meta);
 					break;