diff --git a/tests/functional/qemu_test/cmd.py b/tests/functional/qemu_test/cmd.py
index 98722a9cf6..f6c4e4dda1 100644
--- a/tests/functional/qemu_test/cmd.py
+++ b/tests/functional/qemu_test/cmd.py
@@ -81,6 +81,8 @@ def is_readable_executable_file(path):
 def _console_interaction(test, success_message, failure_message,
                          send_string, keep_sending=False, vm=None):
     assert not keep_sending or send_string
+    assert success_message or send_string
+
     if vm is None:
         vm = test.vm
     console = vm.console_file
@@ -95,7 +97,7 @@ def _console_interaction(test, success_message, failure_message,
                 send_string = None # send only once
 
         # Only consume console output if waiting for something
-        if success_message is None and failure_message is None:
+        if success_message is None:
             if send_string is None:
                 break
             continue
@@ -107,7 +109,7 @@ def _console_interaction(test, success_message, failure_message,
         if not msg:
             continue
         console_logger.debug(msg)
-        if success_message is None or success_message in msg:
+        if success_message in msg:
             break
         if failure_message and failure_message in msg:
             console.close()
@@ -138,6 +140,7 @@ def interrupt_interactive_console_until_pattern(test, success_message,
     :param interrupt_string: a string to send to the console before trying
                              to read a new line
     """
+    assert success_message
     _console_interaction(test, success_message, failure_message,
                          interrupt_string, True)
 
@@ -152,6 +155,7 @@ def wait_for_console_pattern(test, success_message, failure_message=None,
     :param success_message: if this message appears, test succeeds
     :param failure_message: if this message appears, test fails
     """
+    assert success_message
     _console_interaction(test, success_message, failure_message, None, vm=vm)
 
 def exec_command(test, command):
@@ -180,6 +184,7 @@ def exec_command_and_wait_for_pattern(test, command,
     :param success_message: if this message appears, test succeeds
     :param failure_message: if this message appears, test fails
     """
+    assert success_message
     _console_interaction(test, success_message, failure_message, command + '\r')
 
 def get_qemu_img(test):