diff --git a/pa1010d/__init__.py b/pa1010d/__init__.py
index 2a5ac81..98b014e 100644
--- a/pa1010d/__init__.py
+++ b/pa1010d/__init__.py
@@ -79,7 +79,7 @@ def send_command(self, command, add_checksum=True):
         If add_checksum is True (the default) a NMEA checksum will automatically be computed and added.
 
         """
-        if isinstance(command, bytes):
+        if not isinstance(command, bytes):
             command = command.encode("ascii")
 
         # TODO replace with pynmea2 functionality
diff --git a/tests/conftest.py b/tests/conftest.py
index 3881efb..0ab7518 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -26,6 +26,6 @@ def write_byte(self, address, data):
 def smbus():
     smbus = mock.MagicMock()
     smbus.SMBus = SMBus
-    sys.modules["smbus"] = smbus
+    sys.modules["smbus2"] = smbus
     yield smbus
-    del sys.modules["smbus"]
+    del sys.modules["smbus2"]