diff --git a/mistral/tests/unit/engine/test_yaql_functions.py b/mistral/tests/unit/engine/test_yaql_functions.py
index c3a6c6828..76e1d419e 100644
--- a/mistral/tests/unit/engine/test_yaql_functions.py
+++ b/mistral/tests/unit/engine/test_yaql_functions.py
@@ -13,6 +13,7 @@
 #    limitations under the License.
 
 from oslo_config import cfg
+import six
 
 from mistral.db.v2 import api as db_api
 from mistral.services import workflows as wf_service
@@ -431,3 +432,42 @@ class YAQLFunctionsEngineTest(engine_test_base.EngineTestCase):
         self.assertIsNotNone(json_str)
         self.assertIn('"key1": "foo"', json_str)
         self.assertIn('"key2": "bar"', json_str)
+
+    def test_built_in_str_function(self):
+        wf_text = """---
+        version: '2.0'
+
+        wf:
+          input:
+            - my_list
+
+          tasks:
+            task1:
+              publish:
+                val: <% str($.my_list) %>
+        """
+
+        wf_service.create_workflows(wf_text)
+
+        wf_ex = self.engine.start_workflow(
+            'wf',
+            wf_input={
+                'my_list': [
+                    {
+                        'k1': 'v1',
+                        'k2': 'v2'
+                    }
+                ]
+            }
+        )
+
+        self.await_workflow_success(wf_ex.id)
+
+        with db_api.transaction(read_only=True):
+            wf_ex = db_api.get_workflow_execution(wf_ex.id)
+
+            val = wf_ex.task_executions[0].published['val']
+
+            self.assertIsInstance(val, six.string_types)
+            self.assertIn('[', val)
+            self.assertIn(']', val)
diff --git a/mistral/utils/expression_utils.py b/mistral/utils/expression_utils.py
index 1155b10f0..2e4f4035e 100644
--- a/mistral/utils/expression_utils.py
+++ b/mistral/utils/expression_utils.py
@@ -14,6 +14,7 @@
 #    limitations under the License.
 
 from functools import partial
+import six
 import warnings
 
 from oslo_log import log as logging
@@ -39,6 +40,27 @@ LOG = logging.getLogger(__name__)
 ROOT_YAQL_CONTEXT = None
 
 
+def _convert_yaql_input_data(obj, rec=None):
+    # NOTE(rakhmerov): We have to define our own wrapper function
+    # around the function 'convert_input_data' from 'yaql_utils'
+    # because the latter always converts all sequences (except strings)
+    # into tuples, and it in turn breaks a number of things. For example,
+    # if we use the built-in 'str' YAQL function with an argument of the
+    # type 'list' then the result will be '(item1, item2 ..., itemN,)'
+    # instead of '[item1, item2 ..., itemN]'.
+    # So we override this behavior for sequences that are not strings and
+    # tuples.
+    if rec is None:
+        rec = _convert_yaql_input_data
+
+    if (isinstance(obj, yaql_utils.SequenceType) and
+            not isinstance(obj, six.string_types) and
+            not isinstance(obj, tuple)):
+        return list(rec(t, rec) for t in obj)
+    else:
+        return yaql_utils.convert_input_data(obj, rec)
+
+
 def get_yaql_context(data_context):
     global ROOT_YAQL_CONTEXT
 
@@ -48,7 +70,7 @@ def get_yaql_context(data_context):
         _register_yaql_functions(ROOT_YAQL_CONTEXT)
 
     new_ctx = ROOT_YAQL_CONTEXT.create_child_context()
-    new_ctx['$'] = yaql_utils.convert_input_data(data_context)
+    new_ctx['$'] = _convert_yaql_input_data(data_context)
 
     if isinstance(data_context, dict):
         new_ctx['__env'] = data_context.get('__env')