Index: Lib/pstats.py
===================================================================
--- Lib/pstats.py	(revision 76430)
+++ Lib/pstats.py	(working copy)
@@ -522,8 +522,11 @@
         new_callers[func] = caller
     for func, caller in source.iteritems():
         if func in new_callers:
-            new_callers[func] = tuple([i[0] + i[1] for i in
-                                       zip(caller, new_callers[func])])
+            if isinstance(caller, tuple):
+                new_callers[func] = tuple(a + b for a, b in
+                                          zip(caller, new_callers[func]))
+            else:
+                new_callers[func] += caller
         else:
             new_callers[func] = caller
     return new_callers
Index: Lib/test/test_pstats.py
===================================================================
--- Lib/test/test_pstats.py	(revision 76430)
+++ Lib/test/test_pstats.py	(working copy)
@@ -14,6 +14,10 @@
         source = {"a": (1, 2, 3, 4), "b": (5, 6, 7, 8)}
         new_callers = pstats.add_callers(target, source)
         self.assertEqual(new_callers, {'a': (2, 4, 6, 8), 'b': (5, 6, 7, 8)})
+        target = {"a": 1}
+        source = {"a": 1, "b": 5}
+        new_callers = pstats.add_callers(target, source)
+        self.assertEqual(new_callers, {'a': 2, 'b': 5})
 
 
 def test_main():