diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7f6f1c9..31f8cb9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@ All notable changes to this project will be documented in this file.
   * Function `is_valid_syntax_non_zero` (via [#48])
   * Function `is_valid_syntax_pct_encoded` (via [#48])
   * Dict `RFC3987_SYNTAX_TERM_VALIDATORS` mapping `RFC3987_SYNTAX_TERMS` to validator function (via [#49])
+  * Function `is_valid_syntax` issues a warning if an unexpected error occurred (via [#51])
 * Performance
   * Public symbols use lazy loading (via [#43])
 * Tests
@@ -33,6 +34,7 @@ All notable changes to this project will be documented in this file.
 [#48]: https://github.com/jkowalleck/rfc3987-syntax2/pull/48
 [#49]: https://github.com/jkowalleck/rfc3987-syntax2/pull/49
 [#50]: https://github.com/jkowalleck/rfc3987-syntax2/pull/50
+[#51]: https://github.com/jkowalleck/rfc3987-syntax2/pull/51
 [#52]: https://github.com/jkowalleck/rfc3987-syntax2/pull/52
 
 ## v1.2.0 - 2026-07-16
diff --git a/src/rfc3987_syntax2/syntax_helpers.py b/src/rfc3987_syntax2/syntax_helpers.py
index 24a3b59..fd1ebda 100644
--- a/src/rfc3987_syntax2/syntax_helpers.py
+++ b/src/rfc3987_syntax2/syntax_helpers.py
@@ -5,6 +5,7 @@
 from pathlib import Path
 from threading import Lock
 from typing import Any, Callable, Optional, TYPE_CHECKING, Literal
+from warnings import warn
 
 from lark import Lark, ParseTree, exceptions
 
@@ -120,9 +121,17 @@ def parse(term: _T_SYNTAX_PARSER_TERM, value: str) -> 'ParseTree':
 def is_valid_syntax(term: _T_SYNTAX_PARSER_TERM, value: str) -> bool:
     try:
         parse(term=term, value=value)
-        return True
-    except exceptions.LarkError:
+    except exceptions.UnexpectedInput:
+        # from Lark.parse()
         return False
+    except exceptions.LarkError as err:
+        # from Lark internals / initialization (non-UnexpectedInput)
+        warn("Unexpected LarkError (non-UnexpectedInput) "
+             f"for term={term!r}: {type(err).__name__}: {err}",
+            RuntimeWarning,
+            stacklevel=2)
+        return False
+    return True
 
 _T_SYNTAX_VALIDATOR = Callable[[str], bool]
 
@@ -139,9 +148,10 @@ def syntax_validator(text: str) -> bool:
                               parser=RFC3987_SYNTAX_PARSER_TYPE)
         try:
             parser.parse(text)
-            return True
         except exceptions.LarkError:
             return False
+        else:
+            return True
 
     return syntax_validator
 
diff --git a/tests/test_is_valid_syntax_warning.py b/tests/test_is_valid_syntax_warning.py
new file mode 100644
index 0000000..b7c5054
--- /dev/null
+++ b/tests/test_is_valid_syntax_warning.py
@@ -0,0 +1,22 @@
+# Copyright (c) 2026 Jan Kowalleck - modifications and maintenance
+# SPDX-License-Identifier: MIT
+
+import pytest
+from unittest.mock import patch
+
+from lark import exceptions
+
+import rfc3987_syntax2.syntax_helpers as sut
+
+
+def _raise_lark_error(*args: object, **kwargs: object) -> None:
+    raise exceptions.LarkError("unexpected test error")
+
+
+def test_is_valid_syntax_warns_on_unexpected_lark_error() -> None:
+    with patch.object(sut, "parse", side_effect=_raise_lark_error):
+        with pytest.warns(RuntimeWarning, match=r"Unexpected LarkError"):
+            result = sut.is_valid_syntax(
+                term='unsupported-term',  # type: ignore[arg-type] # on purpose
+                value="any-value")
+    assert result is False