diff --git a/subiquity/server/runner.py b/subiquity/server/runner.py
index d196cc4c..f4e602d7 100644
--- a/subiquity/server/runner.py
+++ b/subiquity/server/runner.py
@@ -78,6 +78,8 @@ class LoggedCommandRunner:
     async def wait(self, proc: asyncio.subprocess.Process) \
             -> subprocess.CompletedProcess:
         stdout, stderr = await proc.communicate()
+        # .communicate() forces returncode to be set to a value
+        assert(proc.returncode is not None)
         if proc.returncode != 0:
             raise subprocess.CalledProcessError(proc.returncode, proc.args)
         else:
diff --git a/subiquitycore/utils.py b/subiquitycore/utils.py
index e790cf8d..dcaa86fd 100644
--- a/subiquitycore/utils.py
+++ b/subiquitycore/utils.py
@@ -84,6 +84,8 @@ async def arun_command(cmd: List[str], *,
         if stderr is not None:
             stderr = stderr.decode(encoding)
     log.debug("arun_command %s exited with code %s", cmd, proc.returncode)
+    # .communicate() forces returncode to be set to a value
+    assert(proc.returncode is not None)
     if check and proc.returncode != 0:
         raise subprocess.CalledProcessError(proc.returncode, cmd)
     else: