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Pants

One more :slightly_smiling_face:
```    rule_runner = RuleRunner(rules=rules)
    rule_runner.write_files({"BUILD": "", "knife.utensil": "", "bowl.utensil": ""})
    field_sets = (
        FortranFieldSet(Address("knife"), FortranSource("knife.utensil", Address("knife"))),
        FortranFieldSet(Address("bowl"), FortranSource("bowl.utensil", Address("bowl"))),
    )```
produces:
```E           pants.engine.target.InvalidFieldException: The 'source' field in target knife:knife must have 1 file, but it had 0 files.```

Second address should be empty it seems :open_mouth: