error in parameter setting

I get this error -

Description: F The prefix "fo" for element "fo:block" is not bound.

with this code in my customization stylesheet -

    <xsl:template match="processing-instruction('custom-pagebreak')">
    <fo:block break-before='page'/>
    </xsl:template>

(this is 'supposed' to cause a page break in the .pdf document wherever I 
put <?custom-pagebreak?>)
Is this correct?
What does the error mean and how do I fix this?

Thanks
Chip Wiegand
Computer Services
Simrad, Inc
425-778-8821 
425-771-7211 (FAX)