Error in setting "Content-Security-Policy" Http response header with multiple values

I am trying to set "Content-Security-Policy" Header in following format:

appcmd.exe set config -section:httpProtocol "/+customHeaders.[name='Content-Security-Policy',value='default-src: 'none'; script-src: 'self'; style-src: 'self'; img-src: 'self';']"

But getting error as:

ERROR ( message:Malformed collection indexer; format is [@position,name='value',name2='value2',...].  The @position specifier is optional.

What is correct format for setting this header?