diff --git a/dangerzone/isolation_provider/base.py b/dangerzone/isolation_provider/base.py
index 653be05..04cc7e1 100644
--- a/dangerzone/isolation_provider/base.py
+++ b/dangerzone/isolation_provider/base.py
@@ -94,8 +94,6 @@ class IsolationProvider(ABC):
             self.proc_stderr = subprocess.PIPE
         else:
             self.proc_stderr = subprocess.DEVNULL
-        # Store the proc stderr in memory
-        self.stderr = BytesIO()
 
     def should_capture_stderr(self) -> bool:
         return self.debug or getattr(sys, "dangerzone_dev", False)
@@ -335,8 +333,10 @@ class IsolationProvider(ABC):
         timeout_force: int = TIMEOUT_FORCE,
     ) -> Iterator[subprocess.Popen]:
         """Start a conversion process, pass it to the caller, and then clean it up."""
+        # Store the proc stderr in memory
+        stderr = BytesIO()
         p = self.start_doc_to_pixels_proc(document)
-        stderr_thread = self.start_stderr_thread(p)
+        stderr_thread = self.start_stderr_thread(p, stderr)
 
         if platform.system() != "Windows":
             assert os.getpgid(p.pid) != os.getpgid(
@@ -357,7 +357,7 @@ class IsolationProvider(ABC):
                 # Wait for the thread to complete. If it's still alive, mention it in the debug log.
                 stderr_thread.join(timeout=1)
 
-                debug_bytes = self.stderr.getvalue()
+                debug_bytes = stderr.getvalue()
                 debug_log = read_debug_text(debug_bytes)[:MAX_CONVERSION_LOG_CHARS]
 
                 incomplete = "(incomplete)\n" if stderr_thread.is_alive() else ""
@@ -371,14 +371,14 @@ class IsolationProvider(ABC):
                 )
 
     def start_stderr_thread(
-        self, process: subprocess.Popen
+        self, process: subprocess.Popen, stderr: IO[bytes]
     ) -> Optional[threading.Thread]:
         """Start a thread to read stderr from the process"""
 
         def _stream_stderr(process_stderr: IO[bytes]) -> None:
             try:
                 for line in process_stderr:
-                    self.stderr.write(line)
+                    stderr.write(line)
             except (ValueError, IOError) as e:
                 log.debug(f"Stderr stream closed: {e}")
 
diff --git a/dangerzone/isolation_provider/container.py b/dangerzone/isolation_provider/container.py
index 7200a39..0c3eb59 100644
--- a/dangerzone/isolation_provider/container.py
+++ b/dangerzone/isolation_provider/container.py
@@ -125,7 +125,7 @@ class Container(IsolationProvider):
         with subprocess.Popen(
             [container_runtime, "image", "ls"],
             stdout=subprocess.DEVNULL,
-            stderr=subprocess.PIPE,
+            stderr=self.proc_stderr,
             startupinfo=get_subprocess_startupinfo(),
         ) as p:
             _, stderr = p.communicate()
diff --git a/dangerzone/logic.py b/dangerzone/logic.py
index 0ee17f6..eb588b9 100644
--- a/dangerzone/logic.py
+++ b/dangerzone/logic.py
@@ -1,7 +1,6 @@
 import concurrent.futures
 import json
 import logging
-from io import StringIO
 from typing import Callable, List, Optional
 
 import colorama