diff --git a/mypy/report.py b/mypy/report.py
index 90a62c8a4c7c..398127a33026 100644
--- a/mypy/report.py
+++ b/mypy/report.py
@@ -421,6 +421,9 @@ def on_file(
         type_map: dict[Expression, Type],
         options: Options,
     ) -> None:
+        if os.path.isdir(tree.path):  # can happen with namespace packages
+            return
+
         with open(tree.path) as f:
             tree_source = f.readlines()
 
diff --git a/test-data/unit/reports.test b/test-data/unit/reports.test
index 714610314c88..396414575948 100644
--- a/test-data/unit/reports.test
+++ b/test-data/unit/reports.test
@@ -548,6 +548,14 @@ namespace_packages = True
 </body>
 </html>
 
+[case testReportIsADirectoryErrorCrashOnNamespacePackages]
+# cmd: mypy --linecoverage-report report -p folder
+-- Regression test for https://github.com/python/mypy/issues/18128
+-- "IsADirectoryError for namespace packages when using --linecoverage-report"
+[file folder/subfolder/something.py]
+class Something:
+    pass
+
 [case testReportCoberturaCrashOnNamespacePackages]
 # cmd: mypy --cobertura-xml-report report -p folder
 -- Regression test for https://github.com/python/mypy/issues/19843