diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index e3a0197..e440bea 100644
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -274,7 +274,7 @@
 				if (longflag)
 					sprintf(s, "%ld", va_arg(vargs, long));
 				else if (size_tflag)
-					sprintf(s, "%" PY_FORMAT_SIZE_T "d",
+					sprintf(s, "%" PY_FORMAT_SIZE_T "u",
 					        va_arg(vargs, size_t));
 				else
 					sprintf(s, "%d", va_arg(vargs, int));