diff --git a/src/TensorFlowNET.Core/Operations/OpDefLibrary.cs b/src/TensorFlowNET.Core/Operations/OpDefLibrary.cs
index ab34a320..934ae7d2 100644
--- a/src/TensorFlowNET.Core/Operations/OpDefLibrary.cs
+++ b/src/TensorFlowNET.Core/Operations/OpDefLibrary.cs
@@ -141,6 +141,8 @@ namespace Tensorflow
                             dtype = input_arg.Type;
                         else if (attrs.ContainsKey(input_arg.TypeAttr))
                             dtype = (DataType)attrs[input_arg.TypeAttr];
+                        else if (values.GetType() == typeof(string) && dtype == DataType.DtInvalid)
+                            dtype = DataType.DtString;
                         else if (default_type_attr_map.ContainsKey(input_arg.TypeAttr))
                             default_dtype = (DataType)default_type_attr_map[input_arg.TypeAttr];