diff --git a/TreeConverter/TreeConversion/syntax_tree_visitor.cs b/TreeConverter/TreeConversion/syntax_tree_visitor.cs
index 17a76ed47..9c6a18af1 100644
--- a/TreeConverter/TreeConversion/syntax_tree_visitor.cs
+++ b/TreeConverter/TreeConversion/syntax_tree_visitor.cs
@@ -11608,6 +11608,8 @@ namespace PascalABCCompiler.TreeConverter
                     {
                         if (cnfn.parameters.Count != 1)
                             AddError(new SimpleSemanticError(cnfn.loc,"EXTENSION_METHODS_MUST_HAVE_LEAST_ONE_PARAMETER"));
+                        if (cnfn.ConnectedToType == null)
+                            AddError(new SimpleSemanticError(cnfn.loc, "OPERATOR_SHOULD_BE_EXTENSION_METHOD"));
                         if (!convertion_data_and_alghoritms.eq_type_nodes(tn, cnfn.ConnectedToType) && !convertion_data_and_alghoritms.eq_type_nodes(cnfn.ConnectedToType as type_node, cnfn.parameters[0].type))
                         {
                             AddError(get_location(_function_header.return_type), "RETURN_VALUE_SHOULD_HAVE_TYPE_{0}", (cnfn.ConnectedToType as type_node).PrintableName);