diff --git a/CHANGES.rst b/CHANGES.rst
index bbc92bdd9..fbc683fe1 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -10,6 +10,9 @@ Unreleased
 - Add ``BUILDOUT_HOME`` as an alternate way to control how the user default
   configuration is found.
 
+- Fix edgecase where using multiple inheritance and increments can result in
+  some increments not being included in the final value
+
 2.2.1 (2013-09-05)
 ==================
 
diff --git a/src/zc/buildout/buildout.py b/src/zc/buildout/buildout.py
index 741c47cfe..7f592b1af 100644
--- a/src/zc/buildout/buildout.py
+++ b/src/zc/buildout/buildout.py
@@ -227,6 +227,16 @@ def __init__(self, config_file, cloptions,
         # apply command-line options
         _update(data, cloptions)
 
+        # convert any remaining += to =
+        for sectionname in data:
+            section = data[sectionname]
+            s2 = section.copy()
+            for k, v in s2.items():
+                if k.endswith('+'):
+                    key = k.rstrip(' +')
+                    section[key] = v
+                    del section[k]
+
         # Set up versions section, if necessary
         if 'versions' not in data['buildout']:
             data['buildout']['versions'] = ('versions', 'DEFAULT_VALUE')
@@ -1651,11 +1661,20 @@ def _update_section(s1, s2):
         if k.endswith('+'):
             key = k.rstrip(' +')
             # Find v1 in s2 first; it may have been defined locally too.
-            v1, note1 = s2.get(key, s1.get(key, ("", "")))
+            v1, note1 = s2.get(key, s1.get(key, s1.get(k, ("", ""))))
+            if v1 == '':
+                # merging += in nothing. Keep as +=
+                key = key + " +"
+            elif k in s1:
+                # merging += into +=. keep as +=
+                key = key + " +"
+                del s1[k]
+            else:
+                # merging += into =. convert to =
+                del s2[k]
             newnote = ' [+] '.join((note1, note2)).strip()
             s2[key] = "\n".join((v1).split('\n') +
                 v2.split('\n')), newnote
-            del s2[k]
         elif k.endswith('-'):
             key = k.rstrip(' -')
             # Find v1 in s2 first; it may have been set by a += operation first
diff --git a/src/zc/buildout/tests.py b/src/zc/buildout/tests.py
index 21a1f5328..a93ee233e 100644
--- a/src/zc/buildout/tests.py
+++ b/src/zc/buildout/tests.py
@@ -2671,6 +2671,45 @@ def increment_buildout_with_multiple_extended_files_421022():
       recipe='zc.buildout:debug'
     """
 
+def increment_buildout_with_multiple_extended_without_base_equals():
+    r"""
+
+    >>> write('buildout.cfg', '''
+    ... [buildout]
+    ... extends = base1.cfg base2.cfg
+    ... parts += foo
+    ... [foo]
+    ... recipe = zc.buildout:debug
+    ... [base1]
+    ... recipe = zc.buildout:debug
+    ... [base2]
+    ... recipe = zc.buildout:debug
+    ... ''')
+    >>> write('base1.cfg', '''
+    ... [buildout]
+    ... extends = base3.cfg
+    ... parts += base1
+    ... ''')
+    >>> write('base2.cfg', '''
+    ... [buildout]
+    ... extends = base3.cfg
+    ... parts += base2
+    ... ''')
+    >>> write('base3.cfg', '''
+    ... [buildout]
+    ... ''')
+
+    >>> print_(system(buildout), end='')
+    Installing base1.
+      recipe='zc.buildout:debug'
+    Installing base2.
+      recipe='zc.buildout:debug'
+    Installing foo.
+      recipe='zc.buildout:debug'
+    """
+
+
+
 def increment_on_command_line():
     r"""
     >>> write('buildout.cfg', '''