diff --git a/libagent/gpg/keyring.py b/libagent/gpg/keyring.py
index 343d804..926c01f 100644
--- a/libagent/gpg/keyring.py
+++ b/libagent/gpg/keyring.py
@@ -13,6 +13,7 @@ from .. import util
 
 log = logging.getLogger(__name__)
 
+
 def check_output(args, env=None, sp=subprocess):
     """Call an external binary and return its stdout."""
     log.debug('calling %s with env %s', args, env)
@@ -24,7 +25,7 @@ def check_output(args, env=None, sp=subprocess):
 def get_agent_sock_path(env=None, sp=subprocess):
     """Parse gpgconf output to find out GPG agent UNIX socket path."""
     args = [util.which('gpgconf'), '--list-dirs']
-    output = check_output(args=args, env=env)
+    output = check_output(args=args, env=env, sp=sp)
     lines = output.strip().split(b'\n')
     dirs = dict(line.split(b':', 1) for line in lines)
     log.debug('%s: %s', args, dirs)
@@ -34,7 +35,8 @@ def get_agent_sock_path(env=None, sp=subprocess):
 def connect_to_agent(env=None, sp=subprocess):
     """Connect to GPG agent's UNIX socket."""
     sock_path = get_agent_sock_path(sp=sp, env=env)
-    check_output(args=['gpg-connect-agent', '/bye'])  # Make sure it's running
+    # Make sure the original gpg-agent is running.
+    check_output(args=['gpg-connect-agent', '/bye'], sp=sp)
     sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
     sock.connect(sock_path)
     return sock
@@ -151,7 +153,7 @@ def sign_digest(sock, keygrip, digest, sp=subprocess, environ=None):
 
     assert communicate(sock, 'RESET').startswith(b'OK')
 
-    ttyname = check_output(args=['tty']).strip()
+    ttyname = check_output(args=['tty'], sp=sp).strip()
     options = ['ttyname={}'.format(ttyname)]  # set TTY for passphrase entry
 
     display = (environ or os.environ).get('DISPLAY')
@@ -188,7 +190,8 @@ def sign_digest(sock, keygrip, digest, sp=subprocess, environ=None):
 
 def get_gnupg_components(sp=subprocess):
     """Parse GnuPG components' paths."""
-    output = check_output(args=[util.which('gpgconf'), '--list-components'])
+    args = [util.which('gpgconf'), '--list-components']
+    output = check_output(args=args, sp=sp)
     components = dict(re.findall('(.*):.*:(.*)', output.decode('ascii')))
     log.debug('gpgconf --list-components: %s', components)
     return components
@@ -214,14 +217,14 @@ def gpg_command(args, env=None):
 def get_keygrip(user_id, sp=subprocess):
     """Get a keygrip of the primary GPG key of the specified user."""
     args = gpg_command(['--list-keys', '--with-keygrip', user_id])
-    output = check_output(args=args).decode('ascii')
+    output = check_output(args=args, sp=sp).decode('ascii')
     return re.findall(r'Keygrip = (\w+)', output)[0]
 
 
 def gpg_version(sp=subprocess):
     """Get a keygrip of the primary GPG key of the specified user."""
     args = gpg_command(['--version'])
-    output = check_output(args=args)
+    output = check_output(args=args, sp=sp)
     line = output.split(b'\n')[0]  # b'gpg (GnuPG) 2.1.11'
     return line.split(b' ')[-1]  # b'2.1.11'
 
@@ -229,7 +232,7 @@ def gpg_version(sp=subprocess):
 def export_public_key(user_id, env=None, sp=subprocess):
     """Export GPG public key for specified `user_id`."""
     args = gpg_command(['--export', user_id])
-    result = check_output(args=args, env=env)
+    result = check_output(args=args, env=env, sp=sp)
     if not result:
         log.error('could not find public key %r in local GPG keyring', user_id)
         raise KeyError(user_id)
@@ -239,7 +242,7 @@ def export_public_key(user_id, env=None, sp=subprocess):
 def export_public_keys(env=None, sp=subprocess):
     """Export all GPG public keys."""
     args = gpg_command(['--export'])
-    result = check_output(args=args, env=env)
+    result = check_output(args=args, env=env, sp=sp)
     if not result:
         raise KeyError('No GPG public keys found at env: {!r}'.format(env))
     return result