diff --git a/activesupport/lib/active_support/json/encoding.rb b/activesupport/lib/active_support/json/encoding.rb
index b2adfea273c..e0dee0e072a 100644
--- a/activesupport/lib/active_support/json/encoding.rb
+++ b/activesupport/lib/active_support/json/encoding.rb
@@ -182,6 +182,10 @@ class Numeric
   def encode_json(encoder) to_s end #:nodoc:
 end
 
+class Float
+  def as_json(options = nil) finite? ? self : NilClass::AS_JSON end #:nodoc:
+end
+
 class BigDecimal
   # A BigDecimal would be naturally represented as a JSON number. Most libraries,
   # however, parse non-integer JSON numbers directly as floats. Clients using
diff --git a/activesupport/test/json/encoding_test.rb b/activesupport/test/json/encoding_test.rb
index a2e61d88d5b..8493f4dc2c4 100644
--- a/activesupport/test/json/encoding_test.rb
+++ b/activesupport/test/json/encoding_test.rb
@@ -27,6 +27,9 @@ class TestJSONEncoding < ActiveSupport::TestCase
   NilTests      = [[ nil,   %(null)  ]]
   NumericTests  = [[ 1,     %(1)     ],
                    [ 2.5,   %(2.5)   ],
+                   [ 0.0/0.0,   %(null) ],
+                   [ 1.0/0.0,   %(null) ],
+                   [ -1.0/0.0,  %(null) ],
                    [ BigDecimal('2.5'), %("#{BigDecimal('2.5').to_s}") ]]
 
   StringTests   = [[ 'this is the <string>',     %("this is the \\u003Cstring\\u003E")],