SQL parse xml as table

I have some xml like:

<MyDetails>
  <detail key="key1" value="value1" />
  <detail key="key2" value="value2" />
  <detail key="key3" value="value3" />
  <detail key="key4" value="value4" />
</MyDetails>

And I want to be able to parse it in a table format of two columns Key, and Value. How can I create a SQL function to achieve this by specifying a Node path i.e. in this case '/MyDetails/detail' and KeyAttributeName and ValueAttributeName. I created the following function but it gives me the error:

CREATE FUNCTION GetXmlTable (
     @XmlSource XML
    ,@HierarchyPath NVARCHAR(50)
    ,@SpecificKey NVARCHAR(255) = NULL
    ,@KeyAttributeName NVARCHAR(50) = 'key'
    ,@ValueAttributeName NVARCHAR(50) = 'value'
    )
RETURNS TABLE
AS
RETURN 
(
    SELECT XmlElement.Attribute.value('@' + @KeyAttributeName, 'nvarchar(255)') AS [Key] 
          ,XmlElement.Attribute.value('@' + @ValueAttributeName, 'nvarchar(500)') AS [Value]
    FROM @XmlSource.nodes('/' + @HierarchyPath) AS XmlElement(Attribute)
    where @SpecificKey IS NULL OR XmlElement.Attribute.value('@' + @KeyAttributeName, 'nvarchar(255)') = @SpecificKey
)

GO

Error:

Msg 8172, Level 16, State 1, Procedure GetXmlTable, Line 12 The argument 1 of the XML data type method "nodes" must be a string literal.

Thanks in advance