diff --git a/artiq/coredevice/comm_kernel.py b/artiq/coredevice/comm_kernel.py
index 3d5b8dea9..d37173bbc 100644
--- a/artiq/coredevice/comm_kernel.py
+++ b/artiq/coredevice/comm_kernel.py
@@ -14,6 +14,12 @@ from sipyco.keepalive import create_connection
 logger = logging.getLogger(__name__)


+def format_core_exception(message, params):
+    for i, p in enumerate(params):
+        message = message.replace("{" + str(i) + "}", str(p))
+    return message
+
+
 class Request(Enum):
     SystemInfo = 3

@@ -687,8 +693,7 @@ class CommKernel:
             python_exn_type = embedding_map.retrieve_object(core_exn.id)

         try:
-            python_exn = python_exn_type(
-                nested_exceptions[-1][1].format(*nested_exceptions[0][2]))
+            python_exn = python_exn_type(format_core_exception(nested_exceptions[-1][1], nested_exceptions[0][2]))
         except Exception as ex:
             python_exn = RuntimeError(
                 f"Exception type={python_exn_type}, which couldn't be "
diff --git a/artiq/coredevice/exceptions.py b/artiq/coredevice/exceptions.py
index 7b6967743..ffaae1d36 100644
--- a/artiq/coredevice/exceptions.py
+++ b/artiq/coredevice/exceptions.py
@@ -14,6 +14,12 @@ RuntimeError = builtins.RuntimeError
 AssertionError = builtins.AssertionError


+def format_core_exception(message, params):
+    for i, p in enumerate(params):
+        message = message.replace("{" + str(i) + "}", str(p))
+    return message
+
+
 class CoreException:
     """Information about an exception raised or passed through the core device."""
     def __init__(self, exceptions, exception_info, traceback, stack_pointers):
@@ -70,14 +76,13 @@ class CoreException:
                           self.stack_pointers[start_backtrace_index:]))
         exception = self.exceptions[exception_index]
         name = exception[0]
-        message = exception[1]
-        params = exception[2]
+        message = format_core_exception(exception[1], exception[2])
         if ':' in name:
             exn_id, name = name.split(':', 2)
             exn_id = int(exn_id)
         else:
             exn_id = 0
-        lines.append("{}({}): {}".format(name, exn_id, message.format(*params)))
+        lines.append("{}({}): {}".format(name, exn_id, message))
         zipped.append(((exception[3], exception[4], exception[5], exception[6],
                        None, []), None))