diff --git a/google-apis-core/lib/google/apis/core/composite_io.rb b/google-apis-core/lib/google/apis/core/composite_io.rb
index 9f703a0d5cd..62e662eedbe 100644
--- a/google-apis-core/lib/google/apis/core/composite_io.rb
+++ b/google-apis-core/lib/google/apis/core/composite_io.rb
@@ -25,6 +25,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# frozen_string_literal: true
+
 require 'google/apis/core/http_command'
 require 'google/apis/core/upload'
 require 'google/apis/core/download'
@@ -42,7 +44,7 @@ def initialize(*ios)
         end
 
         def read(length = nil, buf = nil)
-          buf = buf ? buf.replace('') : ''
+          buf = buf ? buf.replace('') : +''
 
           begin
             io = @ios[@index]
diff --git a/google-apis-core/lib/google/apis/core/multipart.rb b/google-apis-core/lib/google/apis/core/multipart.rb
index d176ce9ecfb..74ad1423d0d 100644
--- a/google-apis-core/lib/google/apis/core/multipart.rb
+++ b/google-apis-core/lib/google/apis/core/multipart.rb
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# frozen_string_literal: true
 
 module Google
   module Apis
@@ -31,7 +32,7 @@ def initialize(value, header = {})
         end
 
         def to_io(boundary)
-          part = ''
+          part = +''
           part << "--#{boundary}\r\n"
           part << "Content-Type: application/json\r\n"
           @header.each do |(k, v)|
@@ -59,7 +60,7 @@ def initialize(io, header = {})
         end
 
         def to_io(boundary)
-          head = ''
+          head = +''
           head << "--#{boundary}\r\n"
           @header.each do |(k, v)|
             head << "#{k}: #{v}\r\n"
diff --git a/google-apis-core/spec/google/apis/core/batch_spec.rb b/google-apis-core/spec/google/apis/core/batch_spec.rb
index bd15b49367e..92293a336ad 100644
--- a/google-apis-core/spec/google/apis/core/batch_spec.rb
+++ b/google-apis-core/spec/google/apis/core/batch_spec.rb
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# frozen_string_literal: true
+
 require 'spec_helper'
 require 'google/apis/core/batch'
 require 'google/apis/core/json_representation'
diff --git a/google-apis-core/spec/google/apis/core/service_spec.rb b/google-apis-core/spec/google/apis/core/service_spec.rb
index 53167a12700..061faf9e322 100644
--- a/google-apis-core/spec/google/apis/core/service_spec.rb
+++ b/google-apis-core/spec/google/apis/core/service_spec.rb
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# frozen_string_literal: true
+
 require 'spec_helper'
 require 'google/apis/options'
 require 'google/apis/core/base_service'
diff --git a/google-apis-core/spec/google/apis/core/upload_spec.rb b/google-apis-core/spec/google/apis/core/upload_spec.rb
index 097eb893ef7..c1122e0c53a 100644
--- a/google-apis-core/spec/google/apis/core/upload_spec.rb
+++ b/google-apis-core/spec/google/apis/core/upload_spec.rb
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# frozen_string_literal: true
+
 require 'spec_helper'
 require 'google/apis/core/upload'
 require 'google/apis/core/json_representation'