diff --git a/include/sqlpp11/result_row.h b/include/sqlpp11/result_row.h
index f9e21fcf..628edc76 100644
--- a/include/sqlpp11/result_row.h
+++ b/include/sqlpp11/result_row.h
@@ -265,6 +265,21 @@ namespace sqlpp
 				}
 			}
 	};
+
+	template<typename T>
+		struct is_static_result_row_impl
+		{
+			using type = std::false_type;
+		};
+
+	template<typename Db, typename... FieldSpecs>
+		struct is_static_result_row_impl<result_row_t<Db, FieldSpecs...>>
+		{
+			using type = std::true_type;
+		};
+
+	template<typename T>
+		using is_static_result_row_t = typename is_static_result_row_impl<T>::type;
 }
 
 #endif
diff --git a/include/sqlpp11/union.h b/include/sqlpp11/union.h
index d68ab63b..575f2b08 100644
--- a/include/sqlpp11/union.h
+++ b/include/sqlpp11/union.h
@@ -33,6 +33,7 @@
 #include <sqlpp11/expression.h>
 #include <sqlpp11/interpret_tuple.h>
 #include <sqlpp11/interpretable_list.h>
+#include <sqlpp11/result_row.h>
 #include <sqlpp11/logic.h>
 
 namespace sqlpp
@@ -176,9 +177,12 @@ namespace sqlpp
 					{
 						static_assert(is_statement_t<Rhs>::value, "argument of union call has to be a statement");
 						static_assert(has_policy_t<Rhs, is_select_t>::value, "argument of union call has to be a select");
-						static_assert(has_result_row_t<Rhs>::value, "argument of a union has to be a (complete) select statement");
-						static_assert(has_result_row_t<derived_statement_t<Policies>>::value, "left hand side argument of a union has to be a (complete) select statement");
-						static_assert(std::is_same<get_result_row_t<derived_statement_t<Policies>>, get_result_row_t<Rhs>>::value, "both select statements in a union have to have the same result columns (type and name)");
+						static_assert(has_result_row_t<Rhs>::value, "argument of a union has to be a complete select statement");
+						static_assert(has_result_row_t<derived_statement_t<Policies>>::value, "left hand side argument of a union has to be a complete select statement or union");
+						static_assert(std::is_same<get_result_row_t<derived_statement_t<Policies>>, get_result_row_t<Rhs>>::value, "both arguments in a union have to have the same result columns (type and name)");
+
+						using _result_row_t = get_result_row_t<Rhs>;
+						static_assert(is_static_result_row_t<_result_row_t>::value, "unions must not have dynamically added columns");
 
 						return _union_impl<void, distinct_t>(_check<derived_statement_t<Policies>, Rhs>{}, rhs);
 					}