diff --git a/Modules/structmodule.c b/Modules/structmodule.c
index 2fa6e90..4713c0c 100644
--- a/Modules/structmodule.c
+++ b/Modules/structmodule.c
@@ -1134,7 +1134,7 @@
 				if (n > 255)
 					n = 255;
 				/* store the length byte */
-				*res++ = Py_SAFE_DOWNCAST(n, Py_ssize_t, char);
+				*res++ = Py_SAFE_DOWNCAST(n, Py_ssize_t, unsigned char);
 				res += num;
 				break;
 			}