diff --git a/mistral/tests/unit/utils/test_inspect_utils.py b/mistral/tests/unit/utils/test_inspect_utils.py
index 8ee122ae..67b6650d 100644
--- a/mistral/tests/unit/utils/test_inspect_utils.py
+++ b/mistral/tests/unit/utils/test_inspect_utils.py
@@ -12,6 +12,8 @@
 #    See the License for the specific language governing permissions and
 #    limitations under the License.
 
+import time
+
 from mistral.actions import std_actions
 from mistral.tests.unit import base
 from mistral.utils import inspect_utils as i_u
@@ -37,3 +39,11 @@ class InspectUtilsTest(base.BaseTest):
         parameters_str = i_u.get_arg_list_as_str(clazz.__init__)
 
         self.assertEqual("wf_ex, task_spec, ctx", parameters_str)
+
+    def test_get_parameters_str_with_function_parameter(self):
+
+        def test_func(foo, bar=None, test_func=time.sleep):
+            pass
+
+        parameters_str = i_u.get_arg_list_as_str(test_func)
+        self.assertEqual("foo, bar=null", parameters_str)
diff --git a/mistral/utils/inspect_utils.py b/mistral/utils/inspect_utils.py
index e3491881..407049e5 100644
--- a/mistral/utils/inspect_utils.py
+++ b/mistral/utils/inspect_utils.py
@@ -66,12 +66,15 @@ def get_arg_list_as_str(func):
 
     for index, default in enumerate(args):
         if index >= diff_args_defs:
-            arg_str_list.append(
-                "%s=%s" % (
-                    args[index],
-                    json.dumps(defs[index - diff_args_defs])
+            try:
+                arg_str_list.append(
+                    "%s=%s" % (
+                        args[index],
+                        json.dumps(defs[index - diff_args_defs])
+                    )
                 )
-            )
+            except TypeError:
+                pass
         else:
             arg_str_list.append("%s" % args[index])