diff --git a/lib/cose/key/base.rb b/lib/cose/key/base.rb
index 6b39d17..de48961 100644
--- a/lib/cose/key/base.rb
+++ b/lib/cose/key/base.rb
@@ -41,14 +41,12 @@ def serialize
       end
 
       def map
-        map = {
+        {
           LABEL_BASE_IV => base_iv,
           LABEL_KEY_OPS => key_ops,
           LABEL_ALG => alg,
           LABEL_KID => kid,
-        }
-
-        map.reject { |_k, v| v.nil? }
+        }.compact
       end
     end
   end
diff --git a/lib/cose/key/curve_key.rb b/lib/cose/key/curve_key.rb
index e1b09eb..2266690 100644
--- a/lib/cose/key/curve_key.rb
+++ b/lib/cose/key/curve_key.rb
@@ -35,13 +35,11 @@ def initialize(crv:, x: nil, d: nil, **keyword_arguments) # rubocop:disable Nami
       end
 
       def map
-        map = super.merge(
+        super.merge(
           LABEL_CRV => crv,
           LABEL_X => x,
           LABEL_D => d
-        )
-
-        map.reject { |_k, v| v.nil? }
+        ).compact
       end
     end
   end
diff --git a/lib/cose/key/ec2.rb b/lib/cose/key/ec2.rb
index 021a114..a9c8b0b 100644
--- a/lib/cose/key/ec2.rb
+++ b/lib/cose/key/ec2.rb
@@ -59,12 +59,10 @@ def initialize(y: nil, **keyword_arguments) # rubocop:disable Naming/MethodParam
       end
 
       def map
-        map = super.merge(
+        super.merge(
           Base::LABEL_KTY => KTY_EC2,
           LABEL_Y => y,
-        )
-
-        map.reject { |_k, v| v.nil? }
+        ).compact
       end
 
       def to_pkey
diff --git a/lib/cose/key/rsa.rb b/lib/cose/key/rsa.rb
index 7d63187..2c9d8f0 100644
--- a/lib/cose/key/rsa.rb
+++ b/lib/cose/key/rsa.rb
@@ -74,7 +74,7 @@ def initialize(n:, e:, d: nil, p: nil, q: nil, dp: nil, dq: nil, qinv: nil, **ke
       end
 
       def map
-        map = super.merge(
+        super.merge(
           Base::LABEL_KTY => KTY_RSA,
           LABEL_N => n,
           LABEL_E => e,
@@ -84,9 +84,7 @@ def map
           LABEL_DP => dp,
           LABEL_DQ => dq,
           LABEL_QINV => qinv
-        )
-
-        map.reject { |_k, v| v.nil? }
+        ).compact
       end
 
       def to_pkey