diff --git a/ext/decoders/native/astc.c b/ext/decoders/native/astc.c
index 628666f..3c6928b 100644
--- a/ext/decoders/native/astc.c
+++ b/ext/decoders/native/astc.c
@@ -54,7 +54,7 @@ static inline int getbits(const uint8_t *buf, const int bit, const int len) {
 }
 
 static inline uint_fast64_t getbits64(const uint8_t *buf, const int bit, const int len) {
-    uint_fast64_t mask = len == 64 ? -1 : (1ull << len) - 1;
+    uint_fast64_t mask = len == 64 ? 0xffffffffffffffff : (1ull << len) - 1;
     if (len < 1)
         return 0;
     else if (bit >= 64)
diff --git a/ext/decoders/native/extconf.rb b/ext/decoders/native/extconf.rb
index e93467e..49f6ecf 100644
--- a/ext/decoders/native/extconf.rb
+++ b/ext/decoders/native/extconf.rb
@@ -1,5 +1,6 @@
 require 'mkmf'
 
+append_cppflags('-std=c11')
 append_cppflags('-O3')
 append_cppflags('-Wall')
 append_cppflags('-Wextra')