diff --git a/lib/json_spec/errors.rb b/lib/json_spec/errors.rb
index 6c78388..39d23ae 100644
--- a/lib/json_spec/errors.rb
+++ b/lib/json_spec/errors.rb
@@ -31,4 +31,16 @@ def to_s
       "No JSON file at #{path}"
     end
   end
+
+  class EnumerableExpected < Error
+    attr_reader :actual_value
+
+    def initialize(actual_value)
+      @actual_value = actual_value
+    end
+
+    def to_s
+      "Enumerable expected, got #{actual_value.inspect}"
+    end
+  end
 end
diff --git a/lib/json_spec/matchers/have_json_size.rb b/lib/json_spec/matchers/have_json_size.rb
index 53e829b..e9e11c8 100644
--- a/lib/json_spec/matchers/have_json_size.rb
+++ b/lib/json_spec/matchers/have_json_size.rb
@@ -10,7 +10,8 @@ def initialize(size)
 
       def matches?(json)
         ruby = parse_json(json, @path)
-        @actual = Enumerable === ruby ? ruby.size : 1
+        raise EnumerableExpected.new(ruby) unless Enumerable === ruby
+        @actual = ruby.size
         @actual == @expected
       end
 
diff --git a/spec/json_spec/matchers/have_json_size_spec.rb b/spec/json_spec/matchers/have_json_size_spec.rb
index 92ea547..8b9f357 100644
--- a/spec/json_spec/matchers/have_json_size_spec.rb
+++ b/spec/json_spec/matchers/have_json_size_spec.rb
@@ -46,4 +46,16 @@
     matcher.matches?(%({"id":1,"json":["spec"]}))
     matcher.description.should == %(have JSON size "1" at path "json")
   end
+
+  it "provides an error when parsing nil" do
+    matcher = have_json_size(0).at_path("json")
+    expect { matcher.matches?(%({"id":1,"json":null})) }.to raise_error(JsonSpec::EnumerableExpected,
+                                                                          "Enumerable expected, got nil")
+  end
+
+  it "provides an error when parsing non-enumerables" do
+    matcher = have_json_size(0).at_path("json")
+    expect { matcher.matches?(%({"id":1,"json":12345})) }.to raise_error(JsonSpec::EnumerableExpected,
+                                                                           "Enumerable expected, got 12345")
+  end
 end