Index: lib/rubygems/source_info_cache.rb
===================================================================
--- lib/rubygems/source_info_cache.rb   (revision 1423)
+++ lib/rubygems/source_info_cache.rb   (working copy)
@@ -74,24 +74,38 @@
       @cache_data = Marshal.load data

       @cache_data.each do |url, sice|
-        next unless Hash === sice
+        next unless sice.is_a?(Hash)
         @dirty = true
-        if sice.key? 'cache' and sice.key? 'size' and
-           Gem::SourceIndex === sice['cache'] and Numeric === sice['size'] then
-          new_sice = Gem::SourceInfoCacheEntry.new sice['cache'], sice['size']
+        cache = sice['cache']
+        size  = sice['size']
+        if cache.is_a?(Gem::SourceIndex) and size.is_a?(Numeric) then
+          new_sice = Gem::SourceInfoCacheEntry.new cache, size
           @cache_data[url] = new_sice
         else # irreperable, force refetch.
-          sice = Gem::SourceInfoCacheEntry.new Gem::SourceIndex.new, 0
-          sice.refresh url # HACK may be unnecessary, see ::cache and #refresh
-          @cache_data[url] = sice
+          reset_cache_data(url)
         end
       end
       @cache_data
-    rescue
-      {}
+    rescue => ex
+      if Gem.configuration.really_verbose
+        say "Exception during cache_data handling: #{ex.class} - #{ex}"
+        say "Cache file was: #{cache_file}"
+      end
+      reset_cache_data
     end
   end

+  def reset_cache_data(url = nil)
+    if url
+      sice = Gem::SourceInfoCacheEntry.new Gem::SourceIndex.new, 0
+      sice.refresh url # HACK may be unnecessary, see ::cache and #refresh
+      @cache_data[url] = sice
+    else
+      @cache_data = {}
+    end
+    @cache_data
+  end
+
   # The name of the cache file to be read
   def cache_file
     return @cache_file if @cache_file