diff --git a/tests/data/isa51_tags.json b/tests/data/isa51_tags.json
index 843dc8a..ce1eef7 100644
--- a/tests/data/isa51_tags.json
+++ b/tests/data/isa51_tags.json
@@ -58,17 +58,18 @@
     ["CIC-903", "Indicating Controller", "903"]
   ],
   "invalid": [
-    ["FCI-101", "I"],
-    ["FIH-102", "H"],
-    ["F-103", "function"],
-    ["FICC-104", "repeated"],
-    ["PSVX-105", "X"],
-    ["ABCDEF-106", "D"],
-    ["FQQ-107", "Q"],
-    ["1FC", "format"],
-    ["FIC", "format"],
-    ["fic-101-", "format"],
-    ["LSO-108", "O"]
+    ["FCI-101", "unexpected letter I at position 3"],
+    ["FIH-102", "trailing H needs an alarm (A) or switch (S) before it"],
+    ["F-103", "needs at least one function letter after F"],
+    ["FICC-104", "repeated letter C at position 4"],
+    ["PSVX-105", "unexpected letter X at position 4 after valve V"],
+    ["ABCDE-106", "unexpected letter D at position 4"],
+    ["ABCDEF-106", "not in [area-]LETTERS[-]loop[suffix] format"],
+    ["FQQ-107", "unexpected letter Q at position 3"],
+    ["1FC", "not in [area-]LETTERS[-]loop[suffix] format"],
+    ["FIC", "not in [area-]LETTERS[-]loop[suffix] format"],
+    ["fic-101-", "not in [area-]LETTERS[-]loop[suffix] format"],
+    ["LSO-108", "trailing O (open/closed) is only valid after a switch on a position (Z) tag"]
   ],
   "formats": [
     ["10-FIC-101A", "10", "FIC", "101", "A"],
diff --git a/tests/test_pid_tags.py b/tests/test_pid_tags.py
index 1d80954..0e17177 100644
--- a/tests/test_pid_tags.py
+++ b/tests/test_pid_tags.py
@@ -39,7 +39,23 @@ def test_invalid_tags_name_the_offending_letter(tag, fragment):
     out = parse_tag(tag, kind="instrument")
     assert out["valid"] is False
     assert out["tag"] == tag
-    assert any(fragment in err for err in out["errors"]), out["errors"]
+    # The grammar stops at the first offending letter, so the fixture pins
+    # exactly one error and the fragment must sit in *that* one.
+    assert len(out["errors"]) == 1, out["errors"]
+    assert fragment in out["errors"][0], out["errors"]
+
+
+@pytest.mark.parametrize("tag, fragment", FIXTURE["invalid"])
+def test_invalid_fixture_fragments_bind_the_letter_and_position(tag, fragment):
+    # Every grammar error is prefixed with the tag's own letters, so a fragment
+    # that is a substring of the tag ("I" for FCI) matches whatever the grammar
+    # says and pins nothing. A row must quote the rule text, and a grammar row
+    # must quote the reported position as well as the letter.
+    assert fragment not in tag.upper(), f"{tag!r}: fragment {fragment!r} is part of the tag"
+    if "format" not in fragment:
+        assert "position" in fragment or "letter" in fragment or "trailing" in fragment, (
+            f"{tag!r}: fragment {fragment!r} names neither a letter nor a position"
+        )
 
 
 @pytest.mark.parametrize("tag, area, letters, loop, suffix", FIXTURE["formats"])