diff --git a/src/cysignals/implementation.c b/src/cysignals/implementation.c
index 88dee66..390187f 100644
--- a/src/cysignals/implementation.c
+++ b/src/cysignals/implementation.c
@@ -160,7 +160,7 @@ static inline void print_stderr(const char* s)
 /* str should have enough space allocated */
 static inline void ulong_to_str(unsigned long val, char *str, int base)
 {
-    const char xdigits[16] = "0123456789abcdef";
+    const char xdigits[16] = {'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f'};
     unsigned long aux;
     int len;