diff --git a/src/lxc/utils.c b/src/lxc/utils.c
index 3d0f8641c..91bc802b4 100644
--- a/src/lxc/utils.c
+++ b/src/lxc/utils.c
@@ -2417,7 +2417,7 @@ int parse_byte_size_string(const char *s, int64_t *converted)
 	if (!s || !strcmp(s, ""))
 		return -EINVAL;
 
-	end = stpncpy(dup, s, sizeof(dup));
+	end = stpncpy(dup, s, sizeof(dup) - 1);
 	if (*end != '\0')
 		return -EINVAL;