diff --git a/src/parser.c b/src/parser.c
index 2d70424..4dde7cb 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -3382,7 +3382,7 @@ stringSetPermittedChar(struct data_String *const data, char c, int val)
 static inline int
 stringIsPermittedChar(struct data_String *const data, char c)
 {
-	return data->perm_chars[(unsigned)c];
+	return data->perm_chars[(unsigned char)c];
 }
 static void
 stringAddPermittedCharArr(struct data_String *const data,