bugfix in the bottom-up prediction for the parser

src/runtime/c/pgf/parser.c

@@ -1105,7 +1105,9 @@ pgf_parsing_lookahead(PgfParsing *ps, PgfParseState* state,
 			if (seq->idx != NULL) {
 				PgfLexiconIdxEntry* entry = gu_buf_extend(state->lexicon_idx);
 				entry->idx       = seq->idx;
-				entry->offset    = (current - ps->sentence);
+				entry->offset    =
+					(gu_seq_length(seq->syms) == 0) ? state->start_offset
+					                                : (size_t) (current - ps->sentence);
 			}
 
 			if (len+1 <= max)