diff --git a/src/internal/ptycon_ntaio_impl.c b/src/internal/ptycon_ntaio_impl.c
index c990ff7..da6c720 100644
--- a/src/internal/ptycon_ntaio_impl.c
+++ b/src/internal/ptycon_ntaio_impl.c
@@ -69,7 +69,7 @@ int ptyc_fputs(const char * str, FILE * file)
 			hio,hevent,
 			0,0,&iosb,
 			(void *)str,
-			nbytes,0,0);
+			(uint32_t)nbytes,0,0);
 
 		/* wait */
 		if (status == NT_STATUS_PENDING)