diff --git a/src/hw_ostc_parser.c b/src/hw_ostc_parser.c
index 9782fd7..5e866f6 100644
--- a/src/hw_ostc_parser.c
+++ b/src/hw_ostc_parser.c
@@ -1049,8 +1049,12 @@ hw_ostc_parser_samples_foreach (dc_parser_t *abstract, dc_sample_callback_t call
 				return DC_STATUS_DATAFORMAT;
 			}
 			unsigned int idx = data[offset];
-			if (idx < 1 || idx > parser->ngasmixes) {
-				ERROR(abstract->context, "Invalid gas mix.");
+			if (parser->model == OSTC4 && ccr && idx > parser->nfixed) {
+				// Fix the OSTC4 diluent index.
+				idx -= parser->nfixed;
+			}
+			if (idx < 1 || idx > parser->nfixed) {
+				ERROR(abstract->context, "Invalid gas mix (%u).", idx);
 				return DC_STATUS_DATAFORMAT;
 			}
 			idx--; /* Convert to a zero based index. */