diff --git a/mypy/report.py b/mypy/report.py
index 398127a33026..4a0b965077f6 100644
--- a/mypy/report.py
+++ b/mypy/report.py
@@ -169,11 +169,10 @@ def on_file(
     ) -> None:
         # Count physical lines.  This assumes the file's encoding is a
         # superset of ASCII (or at least uses \n in its line endings).
-        try:
+        if not os.path.isdir(tree.path):  # can happen with namespace packages
             with open(tree.path, "rb") as f:
                 physical_lines = len(f.readlines())
-        except IsADirectoryError:
-            # can happen with namespace packages
+        else:
             physical_lines = 0
 
         func_counter = FuncCounterVisitor()
diff --git a/test-data/unit/reports.test b/test-data/unit/reports.test
index 396414575948..cce2f7295e3b 100644
--- a/test-data/unit/reports.test
+++ b/test-data/unit/reports.test
@@ -548,6 +548,13 @@ namespace_packages = True
 </body>
 </html>
 
+[case testLinecountReportCrashOnNamespacePackages]
+# cmd: mypy --linecount-report report -p folder
+-- Regression test for https://github.com/python/mypy/issues/15979
+[file folder/subfolder/something.py]
+class Something:
+    pass
+
 [case testReportIsADirectoryErrorCrashOnNamespacePackages]
 # cmd: mypy --linecoverage-report report -p folder
 -- Regression test for https://github.com/python/mypy/issues/18128