diff --git a/src/api/api_datatype.cpp b/src/api/api_datatype.cpp
index f1c65b626..5802c1e64 100644
--- a/src/api/api_datatype.cpp
+++ b/src/api/api_datatype.cpp
@@ -102,6 +102,13 @@ extern "C" {
         sort* e;
 
         ptr_vector<constructor_decl> constrs;
+        symbol sname = to_symbol(name);
+
+        if (mk_c(c)->get_dt_plugin()->is_declared(sname)) {
+            SET_ERROR_CODE(Z3_INVALID_ARG, nullptr);
+            RETURN_Z3(nullptr);
+        }
+
         for (unsigned i = 0; i < n; ++i) {
             symbol e_name(to_symbol(enum_names[i]));
             std::string recognizer_s("is_");
@@ -112,8 +119,9 @@ extern "C" {
         }
 
 
+
         {
-            datatype_decl * dt = mk_datatype_decl(dt_util, to_symbol(name), 0, nullptr, n, constrs.data());
+            datatype_decl * dt = mk_datatype_decl(dt_util, sname, 0, nullptr, n, constrs.data());
             bool is_ok = mk_c(c)->get_dt_plugin()->mk_datatypes(1, &dt, 0, nullptr, sorts);
             del_datatype_decl(dt);
 
diff --git a/src/ast/datatype_decl_plugin.cpp b/src/ast/datatype_decl_plugin.cpp
index 041d6740a..d0214d44f 100644
--- a/src/ast/datatype_decl_plugin.cpp
+++ b/src/ast/datatype_decl_plugin.cpp
@@ -832,6 +832,10 @@ namespace datatype {
     bool util::is_declared(sort* s) const {
         return plugin().is_declared(s);
     }
+
+    bool util::is_declared(symbol const& n) const {
+        return plugin().is_declared(n);
+    }
     
     void util::compute_datatype_size_functions(svector<symbol> const& names) {
         map<symbol, status, symbol_hash_proc, symbol_eq_proc> already_found;
@@ -1087,11 +1091,9 @@ namespace datatype {
         sort * datatype = con->get_range();
         def const& dd = get_def(datatype);
         symbol r;
-        for (constructor const* c : dd) {
-            if (c->name() == con->get_name()) {
-                r = c->recognizer();
-            }
-        }
+        for (constructor const* c : dd) 
+            if (c->name() == con->get_name()) 
+                r = c->recognizer();                    
         parameter ps[2] = { parameter(con), parameter(r) };
         d  = m.mk_func_decl(fid(), OP_DT_RECOGNISER, 2, ps, 1, &datatype);
         SASSERT(d);
@@ -1142,17 +1144,15 @@ namespace datatype {
     }
 
     bool util::is_enum_sort(sort* s) {
-        if (!is_datatype(s)) {
-            return false;
-        }
+        if (!is_datatype(s)) 
+            return false;        
         bool r = false;
         if (m_is_enum.find(s, r))
             return r;
         ptr_vector<func_decl> const& cnstrs = *get_datatype_constructors(s);
         r = true;
-        for (unsigned i = 0; r && i < cnstrs.size(); ++i) {
-            r = cnstrs[i]->get_arity() == 0;
-        }
+        for (unsigned i = 0; r && i < cnstrs.size(); ++i) 
+            r = cnstrs[i]->get_arity() == 0;        
         m_is_enum.insert(s, r);
         m_asts.push_back(s);
         return r;
@@ -1284,11 +1284,14 @@ namespace datatype {
         unsigned idx = 0;
         def const& d = get_def(f->get_range());
         for (constructor* c : d) {
-            if (c->name() == f->get_name()) {
-                return idx;
-            }
+            if (c->name() == f->get_name()) 
+                return idx;            
             ++idx;
         }
+        IF_VERBOSE(0, verbose_stream() << f->get_name() << "\n");
+        for (constructor* c : d)
+            IF_VERBOSE(0, verbose_stream() << "!= " << c->name() << "\n");
+        SASSERT(false);
         UNREACHABLE();
         return 0;
     }
diff --git a/src/ast/datatype_decl_plugin.h b/src/ast/datatype_decl_plugin.h
index 4561dfacf..7229636cb 100644
--- a/src/ast/datatype_decl_plugin.h
+++ b/src/ast/datatype_decl_plugin.h
@@ -253,6 +253,7 @@ namespace datatype {
             ptr_vector<constructor> get_constructors(symbol const& s) const;
             ptr_vector<accessor> get_accessors(symbol const& s) const;
             bool is_declared(sort* s) const { return m_defs.contains(datatype_name(s)); }
+            bool is_declared(symbol const& n) const { return m_defs.contains(n); }
             unsigned get_axiom_base_id(symbol const& s) { return m_axiom_bases[s]; }
             util & u() const;
 
@@ -375,6 +376,7 @@ namespace datatype {
         bool is_constructor_of(unsigned num_params, parameter const* params, func_decl* f);
         void reset();
         bool is_declared(sort* s) const;
+        bool is_declared(symbol const& n) const;
         void display_datatype(sort *s, std::ostream& strm);
         bool is_fully_interp(sort * s) const;
         sort_ref_vector datatype_params(sort * s) const;