diff --git a/py/dml/messages.py b/py/dml/messages.py
index 5a277e0c..55358f3a 100644
--- a/py/dml/messages.py
+++ b/py/dml/messages.py
@@ -2150,7 +2150,19 @@ def __init__(self, dmlfile):
         DMLWarning.__init__(self, SimpleSite(dmlfile + ":0"),
                             dmlfile + "ast")
 
+class WDMLAST(DMLWarning):
+    """Some files in the DML standard library are preparsed into files
+    with the `.dmlast` suffix. If such files are encountered
+    elsewhere, then they are ignored to avoid potential coherency
+    problems, and this warning is printed.
+    """
+    fmt = "AST file for file outside the standard library: %s"
+    def __init__(self, dmlfile):
+        DMLWarning.__init__(self, SimpleSite(dmlfile + ":0"),
+                            dmlfile + "ast")
+
 class WWRNSTMT(DMLWarning):
+
     """
     The source code contained a statement "`warning;`", which
     causes a warning to be printed.
diff --git a/py/dml/toplevel.py b/py/dml/toplevel.py
index 8e2e347a..9dd95b2d 100644
--- a/py/dml/toplevel.py
+++ b/py/dml/toplevel.py
@@ -279,10 +279,22 @@ def parse_file(dml_filename):
     ast = parse(contents, file_info, dml_filename, version)
     return ast
 
+class ASTUnpickler(pickle.Unpickler):
+    _safe_classes = {
+        ('dml.ast', 'AST'),
+        ('dml.logging', 'DumpableSite'),
+        ('dml.logging', 'FileInfo')}
+
+    def find_class(self, module, name):
+        if (module, name) not in self._safe_classes:
+            raise pickle.UnpicklingError('broken dmlast data')
+        return super().find_class(module, name)
+
 def load_dmlast(ast_filename):
     '''Return a previously compiled AST, or None'''
     try:
-        return pickle.loads(bz2.BZ2File(ast_filename).read())  # nosec
+        with bz2.BZ2File(ast_filename) as f:
+            return ASTUnpickler(f).load()
     except Exception as e:
         raise ICE(SimpleSite(ast_filename),
                    "Failed to load AST from %r: %s"
@@ -319,6 +331,13 @@ def parse_dmlast_or_dml(dml_filename):
             # error in dml-builtins.dml, one accidentally edits the
             # copy in [host]/bin/dml/, instead of the one in the repo.
             report(WOLDAST(dml_filename))
+        elif (Path(__file__).parent.parent.parent.parent
+              not in Path(ast_filename).parents):
+            # The .dmlast file mechanism is meant only to speed up the
+            # parsing of the standard library. Using it elsewhere would
+            # mean that parser updates can cause confusing errors, and the
+            # speed gains are small, so refuse to do that.
+            report(WDMLAST(dml_filename))
         else:
             file_info, pragmas, parsedata = load_dmlast(ast_filename)
             if file_info.name is None:
@@ -427,8 +446,7 @@ def parse_main_file(inputfilename, explicit_import_path):
 
             deps.setdefault(path, set()).add(importfile)
 
-            path = str(Path(path).resolve())
-            normalized = os.path.normcase(path)
+            normalized = os.path.normcase(str(Path(path).resolve()))
             if normalized in imported:
                 # Already imported
                 if importfile not in imported[normalized]:
diff --git a/test/1.4/errors/T_WDMLAST.dml b/test/1.4/errors/T_WDMLAST.dml
new file mode 100644
index 00000000..c425aff4
--- /dev/null
+++ b/test/1.4/errors/T_WDMLAST.dml
@@ -0,0 +1,10 @@
+/*
+  © 2024 Intel Corporation
+  SPDX-License-Identifier: MPL-2.0
+*/
+dml 1.4;
+
+device test;
+
+/// COMPILE-ONLY
+/// WARNING WDMLAST T_WDMLAST.dml
diff --git a/test/1.4/errors/T_WDMLAST.dmlast b/test/1.4/errors/T_WDMLAST.dmlast
new file mode 100644
index 00000000..e69de29b
diff --git a/test/tests.py b/test/tests.py
index 922ac4ab..bc2246db 100644
--- a/test/tests.py
+++ b/test/tests.py
@@ -1889,6 +1889,8 @@ def test(self):
             # Don't bother.
             'test/1.2/misc/T_dos_newline.dml',
             # empty
+            'test/1.4/errors/T_WDMLAST.dmlast',
+            # empty
             'test/SUITEINFO',
             # data files
             'test/XFAIL',