diff --git a/artiq/compiler/analyses/constness.py b/artiq/compiler/analyses/constness.py
index c6458497f..fbca1ac9b 100644
--- a/artiq/compiler/analyses/constness.py
+++ b/artiq/compiler/analyses/constness.py
@@ -4,7 +4,7 @@ as constant is ever set.
 """
 
 from pythonparser import algorithm, diagnostic
-from .. import types
+from .. import types, builtins
 
 class Constness(algorithm.Visitor):
     def __init__(self, engine):
@@ -23,6 +23,13 @@ class Constness(algorithm.Visitor):
         self.visit(node.slice)
         self.in_assign = old_in_assign
 
+        if self.in_assign and builtins.is_bytes(node.value.type):
+            diag = diagnostic.Diagnostic("error",
+                "type {typ} is not mutable",
+                {"typ": "bytes"},
+                node.loc)
+            self.engine.process(diag)
+
     def visit_AttributeT(self, node):
         old_in_assign, self.in_assign = self.in_assign, False
         self.visit(node.value)
diff --git a/artiq/compiler/testbench/inferencer.py b/artiq/compiler/testbench/inferencer.py
index 3e36d0a38..e7deffd33 100644
--- a/artiq/compiler/testbench/inferencer.py
+++ b/artiq/compiler/testbench/inferencer.py
@@ -3,6 +3,7 @@ from pythonparser import source, diagnostic, algorithm, parse_buffer
 from .. import prelude, types
 from ..transforms import ASTTypedRewriter, Inferencer, IntMonomorphizer, CastMonomorphizer
 from ..transforms import IODelayEstimator
+from ..analyses import Constness
 
 class Printer(algorithm.Visitor):
     """
@@ -83,6 +84,7 @@ def main():
     parsed, comments = parse_buffer(buf, engine=engine)
     typed = ASTTypedRewriter(engine=engine, prelude=prelude.globals()).visit(parsed)
     Inferencer(engine=engine).visit(typed)
+    Constness(engine=engine).visit(typed)
     if monomorphize:
         CastMonomorphizer(engine=engine).visit(typed)
         IntMonomorphizer(engine=engine).visit(typed)
diff --git a/artiq/test/lit/inferencer/error_bytes_subscript_mut.py b/artiq/test/lit/inferencer/error_bytes_subscript_mut.py
new file mode 100644
index 000000000..96abe780d
--- /dev/null
+++ b/artiq/test/lit/inferencer/error_bytes_subscript_mut.py
@@ -0,0 +1,5 @@
+# RUN: %python -m artiq.compiler.testbench.inferencer +diag %s >%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+# CHECK-L: ${LINE:+1}: error: type bytes is not mutable
+(b"a")[0] = 1