diff --git a/src/bindings/lua/context.cpp b/src/bindings/lua/context.cpp
index d42191e15..051064930 100644
--- a/src/bindings/lua/context.cpp
+++ b/src/bindings/lua/context.cpp
@@ -39,9 +39,9 @@ static int context_entry_gc(lua_State * L) {
 static int mk_context_entry(lua_State * L) {
     int nargs = lua_gettop(L);
     if (nargs == 2)
-        return push_context_entry(L, context_entry(to_name_ext(L, 1), to_expr(L, 2)));
+        return push_context_entry(L, context_entry(to_name_ext(L, 1), to_nonnull_expr(L, 2)));
     else
-        return push_context_entry(L, context_entry(to_name_ext(L, 1), to_expr(L, 2), to_expr(L, 3)));
+        return push_context_entry(L, context_entry(to_name_ext(L, 1), to_nonnull_expr(L, 2), to_nonnull_expr(L, 3)));
 }
 
 static int context_entry_pred(lua_State * L) {
@@ -100,9 +100,9 @@ static int mk_context(lua_State * L) {
         context_entry & e = to_context_entry(L, 2);
         return push_context(L, context(to_context(L, 1), e.get_name(), e.get_domain(), e.get_body()));
     } else if (nargs == 3) {
-        return push_context(L, context(to_context(L, 1), to_name_ext(L, 2), to_expr(L, 3)));
+        return push_context(L, context(to_context(L, 1), to_name_ext(L, 2), to_nonnull_expr(L, 3)));
     } else {
-        return push_context(L, context(to_context(L, 1), to_name_ext(L, 2), to_expr(L, 3), to_expr(L, 4)));
+        return push_context(L, context(to_context(L, 1), to_name_ext(L, 2), to_nonnull_expr(L, 3), to_nonnull_expr(L, 4)));
     }
 }
 
diff --git a/src/bindings/lua/expr.cpp b/src/bindings/lua/expr.cpp
index 4b4ebb24f..281441732 100644
--- a/src/bindings/lua/expr.cpp
+++ b/src/bindings/lua/expr.cpp
@@ -29,6 +29,13 @@ expr & to_expr(lua_State * L, int idx) {
     return *static_cast<expr*>(luaL_checkudata(L, idx, expr_mt));
 }
 
+expr & to_nonnull_expr(lua_State * L, int idx) {
+    expr & r = to_expr(L, idx);
+    if (!r)
+        luaL_error(L, "non null Lean expression expected");
+    return r;
+}
+
 int push_expr(lua_State * L, expr const & e) {
     void * mem = lua_newuserdata(L, sizeof(expr));
     new (mem) expr(e);
@@ -79,35 +86,35 @@ static int expr_mk_app(lua_State * L) {
         luaL_error(L, "application must have at least two arguments");
     buffer<expr> args;
     for (int i = 1; i <= nargs; i++)
-        args.push_back(to_expr(L, i));
+        args.push_back(to_nonnull_expr(L, i));
     return push_expr(L, mk_app(args));
 }
 
 static int expr_mk_eq(lua_State * L) {
-    return push_expr(L, mk_eq(to_expr(L, 1), to_expr(L, 2)));
+    return push_expr(L, mk_eq(to_nonnull_expr(L, 1), to_nonnull_expr(L, 2)));
 }
 
 static int expr_mk_lambda(lua_State * L) {
-    return push_expr(L, mk_lambda(to_name_ext(L, 1), to_expr(L, 2), to_expr(L, 3)));
+    return push_expr(L, mk_lambda(to_name_ext(L, 1), to_nonnull_expr(L, 2), to_nonnull_expr(L, 3)));
 }
 
 static int expr_mk_pi(lua_State * L) {
-    return push_expr(L, mk_lambda(to_name_ext(L, 1), to_expr(L, 2), to_expr(L, 3)));
+    return push_expr(L, mk_lambda(to_name_ext(L, 1), to_nonnull_expr(L, 2), to_nonnull_expr(L, 3)));
 }
 
 static int expr_mk_let(lua_State * L) {
     int nargs = lua_gettop(L);
     if (nargs == 3)
-        return push_expr(L, mk_let(to_name_ext(L, 1), expr(), to_expr(L, 2), to_expr(L, 3)));
+        return push_expr(L, mk_let(to_name_ext(L, 1), expr(), to_nonnull_expr(L, 2), to_nonnull_expr(L, 3)));
     else
-        return push_expr(L, mk_let(to_name_ext(L, 1), to_expr(L, 2), to_expr(L, 3), to_expr(L, 4)));
+        return push_expr(L, mk_let(to_name_ext(L, 1), to_nonnull_expr(L, 2), to_nonnull_expr(L, 3), to_nonnull_expr(L, 4)));
 }
 
 static expr get_expr_from_table(lua_State * L, int t, int i) {
     lua_pushvalue(L, t); // push table to the top
     lua_pushinteger(L, i);
     lua_gettable(L, -2);
-    expr r = to_expr(L, -1);
+    expr r = to_nonnull_expr(L, -1);
     lua_pop(L, 2); // remove table and value
     return r;
 }
@@ -140,7 +147,7 @@ int expr_abst(lua_State * L, char const * fname) {
         int len = objlen(L, 1);
         if (len == 0)
             luaL_error(L, "Lean %s expects arg #1 to be non-empty table", fname);
-        expr r = to_expr(L, 2);
+        expr r = to_nonnull_expr(L, 2);
         for (int i = len; i >= 1; i--) {
             auto p = get_expr_pair_from_table(L, 1, i);
             r = F1(p.first, p.second, r);
@@ -149,12 +156,12 @@ int expr_abst(lua_State * L, char const * fname) {
     } else {
         if (nargs % 2 == 0)
             luaL_error(L, "Lean %s must have an odd number of arguments", fname);
-        expr r = to_expr(L, nargs);
+        expr r = to_nonnull_expr(L, nargs);
         for (int i = nargs - 1; i >= 1; i-=2) {
             if (is_expr(L, i - 1))
-                r = F1(to_expr(L, i - 1), to_expr(L, i), r);
+                r = F1(to_nonnull_expr(L, i - 1), to_nonnull_expr(L, i), r);
             else
-                r = F2(to_name_ext(L, i - 1), to_expr(L, i), r);
+                r = F2(to_name_ext(L, i - 1), to_nonnull_expr(L, i), r);
         }
         return push_expr(L, r);
     }
diff --git a/src/bindings/lua/expr.h b/src/bindings/lua/expr.h
index 23e281df8..7bd8beb78 100644
--- a/src/bindings/lua/expr.h
+++ b/src/bindings/lua/expr.h
@@ -11,5 +11,6 @@ class expr;
 void open_expr(lua_State * L);
 bool is_expr(lua_State * L, int idx);
 expr & to_expr(lua_State * L, int idx);
+expr & to_nonnull_expr(lua_State * L, int idx);
 int push_expr(lua_State * L, expr const & o);
 }
diff --git a/src/bindings/lua/local_context.cpp b/src/bindings/lua/local_context.cpp
index 688a9f942..08d54a8f5 100644
--- a/src/bindings/lua/local_context.cpp
+++ b/src/bindings/lua/local_context.cpp
@@ -45,7 +45,7 @@ static int local_entry_mk_lift(lua_State * L) {
 }
 
 static int local_entry_mk_inst(lua_State * L) {
-    return push_local_entry(L, mk_inst(luaL_checkinteger(L, 1), to_expr(L, 2)));
+    return push_local_entry(L, mk_inst(luaL_checkinteger(L, 1), to_nonnull_expr(L, 2)));
 }
 
 static int local_entry_pred(lua_State * L) {