diff --git a/src/unicode.cpp b/src/unicode.cpp
index 665d5b182..ef95cde71 100644
--- a/src/unicode.cpp
+++ b/src/unicode.cpp
@@ -109,7 +109,7 @@ gb_internal isize utf8_decode(u8 const *str, isize str_len, Rune *codepoint_out)
 		u8 b1, b2, b3;
 		Utf8AcceptRange accept;
 		if (x >= 0xf0) {
-			Rune mask = (cast(Rune)x << 31) >> 31;
+			Rune mask = -cast(Rune)(x & 1);
 			codepoint = (cast(Rune)s0 & (~mask)) | (GB_RUNE_INVALID & mask);
 			width = 1;
 			goto end;