diff --git a/.travis.yml b/.travis.yml
index 5fe22ef..86ea484 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -29,7 +29,7 @@ jobs:
       script:
         - black --check -l 120 . --exclude=".*_pb2.py"
         - mypy --ignore-missing-imports --disallow-untyped-defs ./exonum_client ./examples
-        - pylint exonum_client --max-line-length=120 --disable=fixme,bad-continuation,too-few-public-methods; exit $(($? & 3))
+        - pylint exonum_client --max-line-length=120 --disable=fixme,bad-continuation,too-few-public-methods
     - name: docs
       install:
         - pip install sphinx
diff --git a/examples/deploy.py b/examples/deploy.py
index 765fefc..0c339b0 100644
--- a/examples/deploy.py
+++ b/examples/deploy.py
@@ -35,7 +35,7 @@ def run() -> None:
 
             instance_id = start_service(client, service_name, instance_name)
 
-            # If no exception has occured during the previous calls, the service
+            # If no exception has occurred during the previous calls, the service
             # has started successfully:
             print(f"Service instance '{instance_name}' (artifact '{service_name}') started with ID {instance_id}.")
         except RuntimeError as err:
@@ -119,7 +119,7 @@ def send_request(client: ExonumClient, endpoint: str, data: bytes) -> None:
     response = client.post_service("supervisor", endpoint, json_request, private=True)
 
     if response.status_code != 200:
-        error_msg = f"Error occured during the request to the '{endpoint}' endpoint: {response.content}"
+        error_msg = f"Error occurred during the request to the '{endpoint}' endpoint: {response.content!r}"
         raise RuntimeError(error_msg)
 
     # Wait for 10 seconds.
diff --git a/examples/transactions.py b/examples/transactions.py
index 13ca84c..efd359b 100644
--- a/examples/transactions.py
+++ b/examples/transactions.py
@@ -154,7 +154,7 @@ def get_balance(client: ExonumClient, key: PublicKey) -> int:
 def ensure_status_code(response: requests.Response) -> None:
     """Raises an error if the status code is not 200."""
     if response.status_code != 200:
-        raise RuntimeError(f"Received non-ok response: {response.content}")
+        raise RuntimeError(f"Received non-ok response: {response.content!r}")
 
 
 def ensure_transaction_success(client: ExonumClient, tx_hash: str) -> None:
diff --git a/exonum_client/client.py b/exonum_client/client.py
index 8c2a106..44b4a9d 100644
--- a/exonum_client/client.py
+++ b/exonum_client/client.py
@@ -148,12 +148,18 @@ def send_transaction(self, message: ExonumMessage) -> str:
             Result of the WebSocket request.
             If a transaction is correct and it is accepted, it will contain a JSON with a hash of the transaction.
         """
+        body_raw = message.signed_raw()
+        if body_raw is None:
+            logger.critical("Attempt to send an unsigned message through websocket.")
+            raise RuntimeError("Attempt to send an unsigned message.")
+        data = json.dumps({"type": "transaction", "payload": {"tx_body": body_raw.hex()}})
+
         ws_client = WebSocket()
         ws_client.connect(self._address)
-        data = json.dumps({"type": "transaction", "payload": {"tx_body": message.signed_raw().hex()}})
         ws_client.send(data)
         response = ws_client.recv()
         ws_client.close()
+
         return response
 
 
@@ -200,12 +206,12 @@ def __init__(self, hostname: str, public_api_port: int = 80, private_api_port: i
 
     def __repr__(self) -> str:
         """ Conversion to a string. """
-        d = dict()
-
-        d["object"] = f"<{self.__class__.__name__} instance at {id(self)}>"
-        d["host"] = self.hostname
-        d["public_port"] = self.public_api_port
-        d["private_port"] = self.private_api_port
+        d = {
+            "object": f"<{self.__class__.__name__} instance at {id(self)}>",
+            "host": self.hostname,
+            "public_port": str(self.public_api_port),
+            "private_port": str(self.private_api_port),
+        }
 
         return json.dumps(d, indent=2)
 
@@ -634,7 +640,7 @@ def get_peers(self) -> requests.Response:
         """
         return _get(self._system_private_endpoint("peers"))
 
-    def set_consensus_interaction(self, enabled: True) -> requests.Response:
+    def set_consensus_interaction(self, enabled: bool = True) -> requests.Response:
         """
         Performs a POST request to the '{system_base_path}/consensus_enabled'
         to switch consensus interaction of the node on or off.
@@ -716,7 +722,7 @@ def _get_proto_sources(self, params: Optional[Dict[str, str]] = None) -> List[Pr
                 response.status_code,
                 json.dumps(response.json(), indent=2),
             )
-            raise RuntimeError("Unsuccessfully attempted to retrieve Protobuf sources: {}".format(response.content))
+            raise RuntimeError("Unsuccessfully attempted to retrieve Protobuf sources: {!r}".format(response.content))
         logger.debug("Protobuf sources retrieved successfully.")
 
         proto_files = [
diff --git a/exonum_client/proofs/map_proof/errors.py b/exonum_client/proofs/map_proof/errors.py
index f54a3fe..8064fde 100644
--- a/exonum_client/proofs/map_proof/errors.py
+++ b/exonum_client/proofs/map_proof/errors.py
@@ -65,7 +65,7 @@ def malformed_entry(cls, entry: Any, additional_info: Any = None) -> "MalformedM
 
     @classmethod
     def invalid_key_size(cls, key: bytes) -> "MalformedMapProofError":
-        error_msg = f"Invalid key '{key}' for raw MapProof, expected size {KEY_SIZE}, actual {len(key)}"
+        error_msg = f"Invalid key '{key!r}' for raw MapProof, expected size {KEY_SIZE}, actual {len(key)}"
 
         error_data = {"kind": cls.ErrorKind.INVALID_KEY_SIZE, "key": key}
 
diff --git a/requirements.txt b/requirements.txt
index e788b04..8d16442 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -19,7 +19,7 @@ lazy-object-proxy==1.4.2
 MarkupSafe==1.1.1
 mccabe==0.6.1
 more-itertools==7.2.0
-mypy==0.730
+mypy==0.740
 mypy-extensions==0.4.1
 packaging==19.2
 pkginfo==1.5.0.1