diff --git a/HISTORY.md b/HISTORY.md
index 356e3cb..e33b2a9 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -6,7 +6,7 @@ v1.4.0
 ------
 
 * rename model_name to model_class_name for rails 4.2 compatibility
-    
+
     to update, you must generate and run the migration
 
     	bundle exec rails generate reportable_model_name_migration
@@ -51,4 +51,4 @@ v1.0.1
 v1.0.0
 ------
 
-* Initial release of the new Reportable gem (formerly known as the ReportsAsSparkline plugin)
\ No newline at end of file
+* Initial release of the new Reportable gem (formerly known as the ReportsAsSparkline plugin)
diff --git a/README.md b/README.md
index 4bbc386..a1f004b 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,6 @@ Reportable
 
 Reportable allows for the easy creation of reports based on `ActiveRecord` models.
 
-
 Usage
 -----
 
diff --git a/lib/saulabs/reportable/report_cache.rb b/lib/saulabs/reportable/report_cache.rb
index 36bb5fc..012382f 100644
--- a/lib/saulabs/reportable/report_cache.rb
+++ b/lib/saulabs/reportable/report_cache.rb
@@ -86,7 +86,7 @@ def self.process(report, options, &block)
 
         def self.prepare_result(new_data, cached_data, report, options)
           new_data = new_data.to_a.map { |data| [ReportingPeriod.from_db_string(options[:grouping], data[0]), data[1]] }
-          cached_data.to_a.map! { |cached| [ReportingPeriod.new(options[:grouping], cached.reporting_period), cached.value] }
+          cached_data = cached_data.to_a.map { |cached| [ReportingPeriod.new(options[:grouping], cached.reporting_period), cached.value] }
           current_reporting_period = ReportingPeriod.new(options[:grouping])
           reporting_period = get_first_reporting_period(options)
           result = []