diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 39c99fe..41da074 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1485,7 +1485,10 @@
 				num = va_arg(args, long);
 				break;
 			case FORMAT_TYPE_SIZE_T:
-				num = va_arg(args, size_t);
+				if (spec.flags & SIGN)
+					num = va_arg(args, ssize_t);
+				else
+					num = va_arg(args, size_t);
 				break;
 			case FORMAT_TYPE_PTRDIFF:
 				num = va_arg(args, ptrdiff_t);