diff --git a/onigmo/regenc.c b/onigmo/regenc.c
index 16d62fdf4..7be3166ff 100644
--- a/onigmo/regenc.c
+++ b/onigmo/regenc.c
@@ -997,7 +997,8 @@ onigenc_single_byte_ascii_only_case_map(OnigCaseFoldType* flagP, const OnigUChar
 
     if (code >= 'a' && code <= 'z' && (flags & ONIGENC_CASE_UPCASE)) {
       flags |= ONIGENC_CASE_MODIFIED;
-      code += 'A' - 'a';
+      code -= 'a';
+      code += 'A';
     } else if (code >= 'A' && code <= 'Z' &&
 	(flags & (ONIGENC_CASE_DOWNCASE | ONIGENC_CASE_FOLD))) {
       flags |= ONIGENC_CASE_MODIFIED;