15.4.1 Introduction to TAP
http://testanything.org/ Explicitly specifies that a test plan is optional. "The plan The plan tells how many tests will be run, or how many tests have run. It's a check that the test file hasn't stopped prematurely. It must appear once, whether at the beginning or end of the output. The plan is usually the first line of TAP output (although in future there may be a version line before it) and it specifies how many test points are to follow. For example, 1..10 means you plan on running 10 tests. This is a safeguard in case your test file dies silently in the middle of its run. The plan is optional but if there is a plan before the test points it must be the first non-diagnostic line output by the test file. In certain instances a test file may not know how many test points it will ultimately be running. In this case the plan can be the last non-diagnostic line in the output. The plan cannot appear in the middle of the output, nor can it appear more than once." The operand part of this paragraph is " The plan is optional ..." which directly contradicts "It must appear once ...". Your diagnostic message is either incorrect, or more likely, there should be a statement in the Automake Manual providing guidance.
