diff --git a/artiq/compiler/transforms/artiq_ir_generator.py b/artiq/compiler/transforms/artiq_ir_generator.py
index e29949299..f7510516b 100644
--- a/artiq/compiler/transforms/artiq_ir_generator.py
+++ b/artiq/compiler/transforms/artiq_ir_generator.py
@@ -695,9 +695,11 @@ class ARTIQIRGenerator(algorithm.Visitor):
         context_expr_node  = node.items[0].context_expr
         optional_vars_node = node.items[0].optional_vars
 
-        if types.is_builtin(context_expr_node.type, "sequential"):
+        if isinstance(context_expr_node, asttyped.NameT) and \
+                context_expr_node.id == "sequential":
             self.visit(node.body)
-        elif types.is_builtin(context_expr_node.type, "parallel"):
+        elif isinstance(context_expr_node, asttyped.NameT) and \
+                context_expr_node.id == "parallel":
             parallel = self.append(ir.Parallel([]))
 
             heads, tails = [], []
@@ -715,6 +717,9 @@ class ARTIQIRGenerator(algorithm.Visitor):
                 if not tail.is_terminated():
                     tail.append(ir.Branch(self.current_block))
 
+        else:
+            assert False
+
     # Expression visitors
     # These visitors return a node in addition to mutating
     # the IR.
diff --git a/artiq/compiler/transforms/inferencer.py b/artiq/compiler/transforms/inferencer.py
index 8ebe4f5b7..38115fdf2 100644
--- a/artiq/compiler/transforms/inferencer.py
+++ b/artiq/compiler/transforms/inferencer.py
@@ -762,7 +762,11 @@ class Inferencer(algorithm.Visitor):
             self._unify(node.type, typ.find().instance,
                         node.loc, None)
         else:
-            assert False
+            diag = diagnostic.Diagnostic("error",
+                "builtin function '{name}' cannot be used in this context",
+                {"name": typ.name},
+                node.loc)
+            self.engine.process(diag)
 
     def visit_CallT(self, node):
         self.generic_visit(node)
diff --git a/lit-test/test/inferencer/error_illegal_builtin.py b/lit-test/test/inferencer/error_illegal_builtin.py
new file mode 100644
index 000000000..ea8b66a3f
--- /dev/null
+++ b/lit-test/test/inferencer/error_illegal_builtin.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: builtin function 'sequential' cannot be used in this context
+sequential()