XSLT function parameter from xsl:value-of select

I want to substring a particular sentence in XSLT so I use below function and it works fine.

<xsl:value-of select='substring("This is My String",2,6)'/> 

But what I want to do is provide a "This is My String" as a value from XSLT expression like below.

<xsl:value-of select='substring(<xsl:value-of select="searchpage/header/para[2]/text"/>,2,6)'/>

If I do this it gives me below error.

XML Parsing Error: not well-formed

How can i fix this?