diff --git a/gb.h b/gb.h
index c27692d..fd092d3 100644
--- a/gb.h
+++ b/gb.h
@@ -6894,7 +6894,7 @@ isize gb_utf8_decode(u8 const *str, isize str_len, Rune *codepoint_out) {
 		u8 b1, b2, b3;
 		gbUtf8AcceptRange accept;
 		if (x >= 0xf0) {
-			Rune mask = (cast(Rune)x >> 31) << 31;
+			Rune mask = (cast(Rune)x << 31) >> 31;
 			codepoint = (cast(Rune)s0 & (~mask)) | (GB_RUNE_INVALID & mask);
 			width = 1;
 			goto end;