diff --git a/lib/fcm.rb b/lib/fcm.rb
index 3622c77..3a65b7a 100644
--- a/lib/fcm.rb
+++ b/lib/fcm.rb
@@ -4,6 +4,8 @@
 require "googleauth"
 
 class FCM
+  class InvalidCredentialError < StandardError; end
+
   BASE_URI = "https://fcm.googleapis.com"
   BASE_URI_V1 = "https://fcm.googleapis.com/v1/projects/"
   DEFAULT_TIMEOUT = 30
@@ -291,11 +293,31 @@ def jwt_token
     token["access_token"]
   end
 
+  def credentials_error_msg(param)
+    error_msg = 'credentials must be an IO-like ' \
+      'object or path. You passed'
+
+    param_klass = param.nil? ? 'nil' : "a #{param.class.name}"
+    error_msg += " #{param_klass}."
+    raise InvalidCredentialError, error_msg
+  end
+
+  def valid_json_key_path?(path)
+    valid_io_object = path.respond_to?(:open)
+    return true if valid_io_object && File.file?(path)
+
+    max_path_len = 1024
+    valid_path = path.is_a?(String) && path.length <= max_path_len
+    valid_path && File.file?(path)
+  end
+
   def json_key
     @json_key ||= if @json_key_path.respond_to?(:read)
                     @json_key_path
-                  else
+                  elsif valid_json_key_path?(@json_key_path)
                     File.open(@json_key_path)
+                  else
+                    credentials_error_msg(@json_key_path)
                   end
   end
 end
diff --git a/spec/fcm_spec.rb b/spec/fcm_spec.rb
index a3f24e1..4332ba1 100644
--- a/spec/fcm_spec.rb
+++ b/spec/fcm_spec.rb
@@ -1,4 +1,5 @@
 require "spec_helper"
+require 'tempfile'
 
 describe FCM do
   let(:project_name) { 'test-project' }
@@ -13,27 +14,105 @@
     }
   end
 
+  let(:client_email) do
+    '83315528762cf7e0-7bbcc3aad87e0083391bc7f234d487' \
+    'c8@developer.gserviceaccount.com'
+  end
+
+  let(:client_x509_cert_url) do
+    'https://www.googleapis.com/robot/v1/metadata/x509/' \
+    'fd6b61037dd2bb8585527679" + "-7bbcc3aad87e0083391b' \
+    'c7f234d487c8%40developer.gserviceaccount.com'
+  end
+
+  let(:large_file_name) do
+    Array.new(1021) { 'a' }.join('') + '.txt'
+  end
+
+  let(:creds_error) do
+    FCM::InvalidCredentialError
+  end
+
+  let(:json_credentials) do
+    {
+      "type": 'service_account',
+      "project_id": 'example',
+      "private_key_id": 'c09c4593eee53707ca9f4208fbd6fe72b29fc7ab',
+      "private_key": OpenSSL::PKey::RSA.new(2048).to_s,
+      "client_email": client_email,
+      "client_id": 'acedc3c0a63b3562376386f0.apps.googleusercontent.com',
+      "auth_uri": 'https://accounts.google.com/o/oauth2/auth',
+      "token_uri": 'https://oauth2.googleapis.com/token',
+      "auth_provider_x509_cert_url": 'https://www.googleapis.com/oauth2/v1/certs',
+      "client_x509_cert_url": client_x509_cert_url,
+      "universe_domain": 'googleapis.com'
+    }.to_json
+  end
+
   before do
     allow(client).to receive(:json_key)
 
     # Mock the Google::Auth::ServiceAccountCredentials
-    allow(Google::Auth::ServiceAccountCredentials).to receive(:make_creds).
-      and_return(double(fetch_access_token!: { 'access_token' => mock_token }))
+    allow(Google::Auth::ServiceAccountCredentials).to receive(:make_creds)
+      .and_return(double(fetch_access_token!: { 'access_token' => mock_token }))
   end
 
-  it "should initialize" do
+  it 'should initialize' do
     expect { client }.not_to raise_error
   end
 
   describe "credentials path" do
-    it "can be a path to a file" do
+    it 'can be a path to a file' do
       fcm = FCM.new("README.md")
       expect(fcm.__send__(:json_key).class).to eq(File)
     end
 
-    it "can be an IO object" do
-      fcm = FCM.new(StringIO.new("hey"))
+    it 'raises an error when passed a large path' do
+      expect do
+        FCM.new(large_file_name).__send__(:json_key)
+      end.to raise_error(creds_error)
+    end
+
+    it 'can be an IO object' do
+      fcm = FCM.new(StringIO.new('hey'))
       expect(fcm.__send__(:json_key).class).to eq(StringIO)
+
+      temp_file = Tempfile.new('hello_world.json')
+      temp_file.write(json_credentials)
+      fcm_with_temp_file = FCM.new(temp_file)
+
+      expect do
+        fcm_with_temp_file
+      end.not_to raise_error
+      temp_file.close
+      temp_file.unlink
+    end
+
+    it 'raises an error when passed a non IO-like object' do
+      expect do
+        FCM.new(nil, '', {}).__send__(:json_key)
+      end.to raise_error(creds_error, 'credentials must be' \
+      ' an IO-like object or path. You passed nil.')
+
+      expect do
+        FCM.new(json_credentials, '', {}).__send__(:json_key)
+      end.to raise_error(creds_error, 'credentials must be' \
+      ' an IO-like object or path. You passed a String.')
+
+      expect do
+        FCM.new({}, '', {}).__send__(:json_key)
+      end.to raise_error(creds_error, 'credentials must be' \
+      ' an IO-like object or path. You passed a Hash.')
+    end
+
+    it 'raises an error when passed a non-existent credentials file path' do
+      fcm = FCM.new('spec/fake_credentials.json', '', {})
+      expect { fcm.__send__(:json_key) }.to raise_error(creds_error)
+    end
+
+    it 'raises an error when passed a string of a file that does not exist' do
+      fcm = FCM.new('example.txt', '', {})
+      expect { fcm.__send__(:json_key) }.to raise_error(creds_error)
     end
   end