diff --git a/spec/ameba/rule/hash_duplicated_key_spec.cr b/spec/ameba/rule/hash_duplicated_key_spec.cr
index 65a50c2b..4f03e1bd 100644
--- a/spec/ameba/rule/hash_duplicated_key_spec.cr
+++ b/spec/ameba/rule/hash_duplicated_key_spec.cr
@@ -44,7 +44,7 @@ module Ameba::Rule
       )
       subject.catch(s).should_not be_valid
       error = s.errors.first
-      error.message.should eq %(Duplicated keys in hash literal: "key1","key2")
+      error.message.should eq %(Duplicated keys in hash literal: "key1", "key2")
     end
   end
 end
diff --git a/src/ameba/rule/hash_duplicated_key.cr b/src/ameba/rule/hash_duplicated_key.cr
index f8d9fcfa..37a719de 100644
--- a/src/ameba/rule/hash_duplicated_key.cr
+++ b/src/ameba/rule/hash_duplicated_key.cr
@@ -33,7 +33,7 @@ module Ameba::Rule
       return unless (keys = duplicated_keys(node.entries)).any?
 
       source.error self, node.location,
-        "Duplicated keys in hash literal: #{keys.join(",")}"
+        "Duplicated keys in hash literal: #{keys.join(", ")}"
     end
 
     private def duplicated_keys(entries)