Index: Lib/trace.py
===================================================================
--- Lib/trace.py	(revision 87227)
+++ Lib/trace.py	(working copy)
@@ -325,17 +325,18 @@
                 lnotab = _find_executable_linenos(filename)
             else:
                 lnotab = {}
+            if lnotab:
+                source = linecache.getlines(filename)
+                coverpath = os.path.join(dir, modulename + ".cover")
+                with open(filename, 'rb') as fp:
+                    encoding, _ = tokenize.detect_encoding(fp.readline)
+                n_hits, n_lines = self.write_results_file(coverpath, source,
+                                                          lnotab, count, encoding)
+                if summary and n_lines:
+                    percent = int(100 * n_hits / n_lines)
+                    sums[modulename] = n_lines, percent, modulename, filename
+            
 
-            source = linecache.getlines(filename)
-            coverpath = os.path.join(dir, modulename + ".cover")
-            with open(filename, 'rb') as fp:
-                encoding, _ = tokenize.detect_encoding(fp.readline)
-            n_hits, n_lines = self.write_results_file(coverpath, source,
-                                                      lnotab, count, encoding)
-            if summary and n_lines:
-                percent = int(100 * n_hits / n_lines)
-                sums[modulename] = n_lines, percent, modulename, filename
-
         if summary and sums:
             print("lines   cov%   module   (path)")
             for m in sorted(sums):
Index: Lib/test/test_trace.py
===================================================================
--- Lib/test/test_trace.py	(revision 87227)
+++ Lib/test/test_trace.py	(working copy)
@@ -9,7 +9,6 @@
 
 from test.tracedmodules import testmod
 
-
 #------------------------------- Utilities -----------------------------------#
 
 def fix_ext_py(filename):
@@ -220,7 +219,12 @@
     def setUp(self):
         self.tracer = Trace(count=0, trace=0, countfuncs=1)
         self.filemod = my_file_and_modname()
+        self._saved_tracefunc = sys.gettrace()
 
+    def tearDown(self):
+        if self._saved_tracefunc is not None:
+            sys.settrace(self._saved_tracefunc)
+
     def test_simple_caller(self):
         self.tracer.runfunc(traced_func_simple_caller, 1)