diff --git a/src/solvers/sat/external_sat.cpp b/src/solvers/sat/external_sat.cpp
index 94105fcbfbe..eec7b0e729b 100644
--- a/src/solvers/sat/external_sat.cpp
+++ b/src/solvers/sat/external_sat.cpp
@@ -177,6 +177,7 @@ external_satt::resultt external_satt::do_prop_solve()
   // create a temporary file
   temporary_filet cnf_file("external-sat", ".cnf");
   write_cnf_file(cnf_file());
+  clauses.clear();
   auto output = execute_solver(cnf_file());
   return parse_result(output);
 }