diff --git a/src/smt/smt_context.cpp b/src/smt/smt_context.cpp
index 37a6d32b7..50b957331 100644
--- a/src/smt/smt_context.cpp
+++ b/src/smt/smt_context.cpp
@@ -3115,11 +3115,18 @@ namespace smt {
     }
 
     bool is_valid_assumption(ast_manager & m, expr * assumption) {
+        expr* arg;
         if (!m.is_bool(assumption))
             return false;
         if (is_uninterp_const(assumption))
             return true;
-        if (m.is_not(assumption) && is_uninterp_const(to_app(assumption)->get_arg(0)))
+        if (m.is_not(assumption, arg) && is_uninterp_const(arg))
+            return true;
+        if (!is_app(assumption)) 
+            return false;
+        if (to_app(assumption)->get_num_args() == 0)
+            return true;
+        if (m.is_not(assumption, arg) && is_app(arg) && to_app(arg)->get_num_args() == 0)
             return true;
         return false;
     }
diff --git a/src/smt/theory_str.cpp b/src/smt/theory_str.cpp
index 835f3b553..128f93b11 100644
--- a/src/smt/theory_str.cpp
+++ b/src/smt/theory_str.cpp
@@ -7351,10 +7351,10 @@ namespace smt {
 
     void theory_str::add_theory_assumptions(expr_ref_vector & assumptions) {
         TRACE("str", tout << "add overlap assumption for theory_str" << std::endl;);
-        symbol strOverlap("!!TheoryStrOverlapAssumption!!");
+        char* strOverlap = "!!TheoryStrOverlapAssumption!!";
         seq_util m_sequtil(get_manager());
         sort * s = get_manager().mk_bool_sort();
-        m_theoryStrOverlapAssumption_term = expr_ref(get_manager().mk_const(strOverlap, s), get_manager());
+        m_theoryStrOverlapAssumption_term = expr_ref(mk_fresh_const(strOverlap, s), get_manager());
         assumptions.push_back(get_manager().mk_not(m_theoryStrOverlapAssumption_term));
     }