Internal DTD validation using Notepad++

I am new to XML and trying to create a simple XML with DTD.

I am using Notepad++ via XML Tools blugin to validate.

Here is my XML code with internal DTD.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<!DOCTYPE People [
    <!ELEMENT People (name, phone*)>
    <!ELEMENT name (#PCDATA)*>
    <!ELEMENT phone (#PCDATA)*>   
    <!ATTLIST phone type (home) #REQUIRED>
]>

<People>
    <name> John Smith </name>
    <phone type="home">+1 (123) 456-7890 </phone>
</People>

If I keep 'standalone="yes"', I received the following error "ERROR: standalone: People declared in the external subset contains white spaces nodes". If I remove it, the code is validated with no issues.

Can anyone shed some light on why the presence of 'standalone="yes"' causes problems? Thank you.

P.S. please read the description of the above problem thoroughly before mark it as duplicate.

Solution

Validity constraint: Standalone Document Declaration

The standalone document declaration must have the value "no" if any external markup declarations contain declarations of:

attributes with default values, if elements to which these attributes apply appear in the document without specifications of values for these attributes, or

entities (other than amp, lt, gt, apos, quot), if references to those entities appear in the document, or

attributes with tokenized types, where the attribute appears in the document with a value such that normalization will produce a
different value from that which would be produced in the absence of
the declaration, or

element types with element content, if white space occurs directly within any instance of those types.

Reference : Extensible Markup Language (XML) 1.0 (Fifth Edition)