diff --git a/modules/io/src/mol/mmcif_writer.cc b/modules/io/src/mol/mmcif_writer.cc
index 7ffc2fa4ee0dd0cf0779b59f45609052cdef1eba..faaaa5c053947b48f15ed9ccb86bcb98c805fd54 100644
--- a/modules/io/src/mol/mmcif_writer.cc
+++ b/modules/io/src/mol/mmcif_writer.cc
@@ -1338,7 +1338,7 @@ namespace {
       if(entity_infos[entity_idx].is_poly) {
         // scan for "-" in mon_ids
         for(size_t mon_id_idx = 0;
-            mon_id_idx < entity_infos[entity_idx].mon_ids.size(); ++entity_idx) {
+            mon_id_idx < entity_infos[entity_idx].mon_ids.size(); ++mon_id_idx) {
 
           if(entity_infos[entity_idx].mon_ids[mon_id_idx] == "-") {