--- spack-src/alphabet.cpp.bak	2020-07-23 11:52:57.000000000 +0900
+++ spack-src/alphabet.cpp	2020-09-16 17:05:52.093190703 +0900
@@ -274,7 +274,7 @@
 const char *iupacs = "!ACMGRSVTWYHKDBN!acmgrsvtwyhkdbn";
 
 signed char mask2iupac[16] = {
-	-1,
+	static_cast<signed char>(-1),
 	'A', // 0001
 	'C', // 0010
 	'M', // 0011