Odd missing values problem

I've tried adding each of these expressions to my template:

${item.publication!}
${item.publication!""}
${item.publication!"foo"}

For each, I get an error like this:
[ERROR: item.publication! evaluated to null string.]

item.publication returns a TemplateScalarModel, and the .getAsString() 
method returns a null.

What am I doing wrong?

I did add the "missing values" example from the manual to the top of the 
page and it works correctly:

${mouse!"No mouse."}
<#assign mouse="Jerry">
${mouse!"No mouse."}

No mouse.
Jerry


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