diff --git a/interop.py b/interop.py
index 93a83e61..024e64a1 100644
--- a/interop.py
+++ b/interop.py
@@ -140,7 +140,7 @@ def _check_impl_is_compliant(self, name: str) -> bool:
         )
         if not self._is_unsupported(output.stdout.splitlines()):
             logging.error("%s client not compliant.", name)
-            logging.debug("%s", output.stdout.decode("utf-8"))
+            logging.debug("%s", output.stdout.decode("utf-8", errors="replace"))
             self.compliant[name] = False
             return False
         logging.debug("%s client compliant.", name)
@@ -166,7 +166,7 @@ def _check_impl_is_compliant(self, name: str) -> bool:
         )
         if not self._is_unsupported(output.stdout.splitlines()):
             logging.error("%s server not compliant.", name)
-            logging.debug("%s", output.stdout.decode("utf-8"))
+            logging.debug("%s", output.stdout.decode("utf-8", errors="replace"))
             self.compliant[name] = False
             return False
         logging.debug("%s server compliant.", name)
@@ -310,7 +310,9 @@ def _copy_logs(self, container: str, dir: tempfile.TemporaryDirectory):
         )
         if r.returncode != 0:
             logging.info(
-                "Copying logs from %s failed: %s", container, r.stdout.decode("utf-8")
+                "Copying logs from %s failed: %s",
+                container,
+                r.stdout.decode("utf-8", errors="replace"),
             )
 
     def _run_testcase(
@@ -393,7 +395,7 @@ def _run_test(
             output = ex.stdout
             expired = True
 
-        logging.debug("%s", output.decode("utf-8"))
+        logging.debug("%s", output.decode("utf-8", errors="replace"))
 
         if expired:
             logging.debug("Test failed: took longer than %ds.", testcase.timeout())
@@ -404,7 +406,7 @@ def _run_test(
                 stderr=subprocess.STDOUT,
                 timeout=60,
             )
-            logging.debug("%s", r.stdout.decode("utf-8"))
+            logging.debug("%s", r.stdout.decode("utf-8", errors="replace"))
 
         # copy the pcaps from the simulator
         self._copy_logs("sim", sim_log_dir)