diff --git a/tests/ipv6-prefix.c b/tests/ipv6-prefix.c
index e7c99c6c..5cc77cf2 100644
--- a/tests/ipv6-prefix.c
+++ b/tests/ipv6-prefix.c
@@ -23,18 +23,17 @@
 #include "../src/ip-util.h"
 #include "../src/ip-util.c"
 
-static char* my_ipv6_prefix_to_mask(const char *str, unsigned prefix)
+static char* my_ipv6_prefix_to_mask(char str[MAX_IP_STR], unsigned prefix)
 {
 	struct in6_addr in;
-	static char out[64];
 
 	if (ipv6_prefix_to_mask(&in, prefix) == 0)
 		return NULL;
 
-	if (inet_ntop(AF_INET6, &in, out, sizeof(out)) == NULL)
+	if (inet_ntop(AF_INET6, &in, str, MAX_IP_STR) == NULL)
 		return NULL;
 
-	return out;
+	return str;
 }
 
 int main()