Workflow and Field x should be multi-value warning

Using a simple workflow I am trying to do a field update when the Fixed in build field is changed from Next Build to a build number.  I created an Environment enum(1) field with values Development, Test, Production.  When trying to add the workflow to the project it gives a warning that field Environment should be multi-value.  Why would it warn this when I just want one value selected in that field.  Below is the small script for the workflow.

rule sample rule to update field when Fixed in build is changed

when Fixed in build.changed {
  Environment = {Development};
}