diff --git a/sprintf.c b/sprintf.c
index cdc2d2d738..0438b5e32c 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -1301,14 +1301,19 @@ ruby__sfvwrite(register rb_printf_buffer *fp, register struct __suio *uio)
     struct __siov *iov;
     VALUE result = (VALUE)fp->_bf._base;
     char *buf = (char*)fp->_p;
-    size_t len, n;
-    size_t blen = buf - RSTRING_PTR(result), bsiz = fp->_w;
+    long len, n;
+    long blen = buf - RSTRING_PTR(result), bsiz = fp->_w;
 
     if (RBASIC(result)->klass) {
 	rb_raise(rb_eRuntimeError, "rb_vsprintf reentered");
     }
-    if ((len = uio->uio_resid) == 0)
+    if (uio->uio_resid == 0)
 	return 0;
+#if SIZE_MAX > LONG_MAX
+    if (uio->uio_resid >= LONG_MAX)
+	rb_raise(rb_eRuntimeError, "too big string");
+#endif
+    len = (long)uio->uio_resid;
     CHECK(len);
     buf += blen;
     fp->_w = bsiz;