diff --git a/http_parser.c b/http_parser.c
index 98e0b9f2..1c6314fe 100644
--- a/http_parser.c
+++ b/http_parser.c
@@ -1572,7 +1572,7 @@ size_t http_parser_execute (http_parser *parser,
             REEXECUTE();
           }
 
-          if (!lenient && !IS_HEADER_CHAR(ch)) {
+          if (!lenient && !IS_HEADER_CHAR((unsigned char)ch)) {
             SET_ERRNO(HPE_INVALID_HEADER_TOKEN);
             goto error;
           }