diff --git a/src/muz/transforms/dl_mk_filter_rules.cpp b/src/muz/transforms/dl_mk_filter_rules.cpp
index d23da72fb..b112f4c99 100644
--- a/src/muz/transforms/dl_mk_filter_rules.cpp
+++ b/src/muz/transforms/dl_mk_filter_rules.cpp
@@ -111,7 +111,7 @@ namespace datalog {
         bool rule_modified = false;
         for (unsigned i = 0; i < sz; i++) {
             app * tail = r->get_tail(i);
-            if (is_candidate(tail)) {
+            if (is_candidate(tail) && !r->is_neg_tail(i)) {
                 TRACE("mk_filter_rules", tout << "is_candidate: " << mk_pp(tail, m) << "\n";);
                 var_idx_set non_local_vars = rm.collect_rule_vars_ex(r, tail);
                 func_decl * filter_decl = mk_filter_decl(tail, non_local_vars);