diff --git a/src/dom/scanner.c b/src/dom/scanner.c
index 61af5e127..509b047bf 100644
--- a/src/dom/scanner.c
+++ b/src/dom/scanner.c
@@ -131,9 +131,11 @@ init_dom_scanner_info(struct dom_scanner_info *scanner_info)
 		const struct dom_string *data = &info[i].data;
 
 		if (info[i].type == DOM_SCAN_RANGE) {
-			int index = *data->string;
+			int index = (unsigned char)*data->string;
 
+			assert(index > 0);
 			assert(data->length < DOM_SCAN_TABLE_SIZE);
+			assert(index <= data->length);
 
 			for (; index <= data->length; index++)
 				scan_table[index] |= info[i].bits;