diff --git a/src/lib-sieve/plugins/enotify/mailto/uri-mailto.c b/src/lib-sieve/plugins/enotify/mailto/uri-mailto.c
index 334bc62a95faf7bd01c038d105af0c069d4ff88a..c5a0953653a3ecaa2ca454f954bcd2ce8b9a7eb1 100644
--- a/src/lib-sieve/plugins/enotify/mailto/uri-mailto.c
+++ b/src/lib-sieve/plugins/enotify/mailto/uri-mailto.c
@@ -134,9 +134,9 @@ static const char _qchar_lookup[256] = {
 	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // F0
 };
 
-static inline bool _is_qchar(unsigned char c)
+static inline bool _is_qchar(char c)
 {
-	return ((_qchar_lookup[c] & 0x01) != 0);
+	return ((_qchar_lookup[(unsigned char)c] & 0x01) != 0);
 }
 
 static inline int _decode_hex_digit(unsigned char digit)