diff --git a/py/dml/ctree.py b/py/dml/ctree.py
index 62d59f3b..2a466759 100644
--- a/py/dml/ctree.py
+++ b/py/dml/ctree.py
@@ -5167,9 +5167,10 @@ def toc(self):
         path = self.site.filename()
         (ident, h_path) = dmldir_macro(path)
         out('#define %s "%s"\n' % (ident, quote_filename(h_path)))
-        if dml.globals.linemarks:
-            linemark(self.site.lineno, path)
-        out(self.text)
+        for (i, line) in enumerate(self.text.splitlines(keepends=True)):
+            if dml.globals.linemarks:
+                linemark(self.site.lineno + i, path)
+            out(line)
         if not output.current().bol:
             out('\n')
         if dml.globals.linemarks:
diff --git a/py/dml/output.py b/py/dml/output.py
index 27acd5c5..5618adf5 100644
--- a/py/dml/output.py
+++ b/py/dml/output.py
@@ -35,6 +35,7 @@ def __init__(self, indent=0):
         self.bol = True
         self.redirected_filename = None
         self.redirected_lineno = None
+        self.linemark_is_fresh = False
 
     def write(self, s):
         assert False
@@ -55,6 +56,7 @@ def linemark(self, lineno, filename):
                      % (lineno, quote_filename(filename)))
             self.redirected_filename = filename
             self.redirected_lineno = lineno
+        self.linemark_is_fresh = True
 
     def reset_line_directive(self):
         assert self.filename is not None
@@ -72,6 +74,12 @@ def indeterminate_line_directive(self):
 
     def out(self, output, preindent = 0, postindent = 0):
         self.indent += preindent * indent_level
+        if (self.bol and self.redirected_lineno is not None
+            and not self.linemark_is_fresh):
+            raise ICE(SimpleSite(f'{self.redirected_filename}'
+                                 f':{self.redirected_lineno}'),
+                                 'missing linemark')
+        self.linemark_is_fresh = False
         if output == '\n':
             # Don't indent empty lines...
             self.write('\n')
@@ -84,6 +92,10 @@ def out(self, output, preindent = 0, postindent = 0):
             self.write(output)
             self.bol = (output.endswith('\n'))
         no_lines = output.count('\n')
+        if no_lines > 1 and self.redirected_lineno is not None:
+            raise ICE(SimpleSite(f'{self.redirected_filename}'
+                                 f':{self.redirected_lineno}'),
+                                 'missing linemark')
         self.indent += postindent * indent_level
         self.lineno += no_lines
         if self.redirected_lineno is not None:
diff --git a/py/dml/output_test.py b/py/dml/output_test.py
new file mode 100644
index 00000000..5867d4c1
--- /dev/null
+++ b/py/dml/output_test.py
@@ -0,0 +1,19 @@
+import unittest
+
+from dml import output, logging
+
+class TestOutput(unittest.TestCase):
+    def test_linemark_validation(self):
+        out = output.StrOutput()
+        out.linemark(3, 'foo')
+        out.out('foo')
+        # ok, not beginning of line
+        out.out('bar\n')
+        out.linemark(4, 'foo')
+        out.out('foo')
+        out.out('bar\n')
+        with self.assertRaisesRegex(logging.ICE, '.*missing linemark'):
+            out.out('foo')
+        out.linemark(5, 'foo')
+        with self.assertRaisesRegex(logging.ICE, '.*missing linemark'):
+            out.out('foo\nbar\n')