Struts2 debug tag thinks toString() is udefined

Debug tag in Struts2 keeps 开发者_运维技巧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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Never mind. Value stack had an object with overridden toString method that returned null.