Hello,
I'm not able to use an inline variable (i.e., %Value%) to populate the Z value field of the IDW tool associated with the Spatial Analyst (SA) and 3D Analyst (3D) toolboxes. The Z value field of the IDW tool associated with the Geostatistical Analyst (GA) toolbox, however, does accept an inline variable (i.e., %Value%).
Here's the model:

A precondition for all three IDW tools is the Value variable (shown using dotted arrow lines).



The error generated for the IDW (3D) and IDW (SA) tools is:
ERROR 001000
Z value field: Field %Value% does not exist
I need to use either the 3D or SA versions of the IDW tool, because is contains an Input barrier polyline features field; the GA version does not.
Oddly, the SA and 3D IDW versions accept inline variables for the Output Raster field.
Any idea what the issue could be? Bug?
Is there a workaround?
I'm working with ArcGIS version 10.3.1 for Desktop.
Thanks for your time and any help,
Keith