diff --git a/src/cmd_context/cmd_context.cpp b/src/cmd_context/cmd_context.cpp
index b5c99f5ce..6bcca936a 100644
--- a/src/cmd_context/cmd_context.cpp
+++ b/src/cmd_context/cmd_context.cpp
@@ -1123,7 +1123,7 @@ bool cmd_context::try_mk_builtin_app(symbol const & s, unsigned num_args, expr *
         result = m().mk_app(fid, k, num_indices, indices, num_args, args, range);
     }
     CHECK_SORT(result.get());
-    return true;
+    return nullptr != result.get();
 }
 
 bool cmd_context::try_mk_declared_app(symbol const & s, unsigned num_args, expr * const * args, 
diff --git a/src/parsers/smt2/smt2parser.cpp b/src/parsers/smt2/smt2parser.cpp
index 65a6f5ed5..344c61283 100644
--- a/src/parsers/smt2/smt2parser.cpp
+++ b/src/parsers/smt2/smt2parser.cpp
@@ -1944,6 +1944,8 @@ namespace smt2 {
                 expr ** expr_it   = expr_stack().data() + fr->m_expr_spos;
                 expr ** expr_end  = expr_it + num_decls;
                 for (; expr_it != expr_end; ++expr_it, ++sym_it) {
+                    if (!(*expr_it))
+                        throw parser_exception("invalid let expression");
                     TRACE("let_frame", tout << "declaring: " << *sym_it << " " << mk_pp(*expr_it, m()) << "\n";);
                     m_env.insert(*sym_it, local(*expr_it, m_num_bindings));
                 }