Struts2 debug tag thinks toString() is udefined

Question

Debug tag in Struts2 keeps getting this error. Any ideas what could cause this?

FreeMarker template error!

Expression stackObject.value.get(propertyName).toString() is undefined on line 58, column 122 in template/simple/debug.ftl.
The problematic instruction:
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==> ${stackObject.value.get(propertyName).toString()?html} [on line 58, column 120 in template/simple/debug.ftl]
 in include "/${parameters.templateDir}/simple/debug.ftl" [on line 23, column 1 in template/xhtml/debug.ftl]
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Answer 1

Never mind. Value stack had an object with overridden toString method that returned null.