diff --git a/artiq/compiler/transforms/interleaver.py b/artiq/compiler/transforms/interleaver.py
index 873e55eac..4aa3c327b 100644
--- a/artiq/compiler/transforms/interleaver.py
+++ b/artiq/compiler/transforms/interleaver.py
@@ -3,6 +3,8 @@
 the timestamp would always monotonically nondecrease.
 """
 
+from pythonparser import diagnostic
+
 from .. import types, builtins, ir, iodelay
 from ..analyses import domination
 from ..algorithms import inline
@@ -127,6 +129,14 @@ class Interleaver:
                     else: # It's a call.
                         need_to_inline = len(source_blocks) > 1
                         if need_to_inline:
+                            if old_decomp.static_target_function is None:
+                                diag = diagnostic.Diagnostic("fatal",
+                                    "it is not possible to interleave this function call within "
+                                    "a 'with parallel:' statement because the compiler could not "
+                                    "prove that the same function would always be called", {},
+                                    old_decomp.loc)
+                                self.engine.process(diag)
+
                             inline(old_decomp)
                             postdom_tree = domination.PostDominatorTree(func)
                             continue
diff --git a/lit-test/test/interleaving/error_inlining.py b/lit-test/test/interleaving/error_inlining.py
new file mode 100644
index 000000000..c34959a51
--- /dev/null
+++ b/lit-test/test/interleaving/error_inlining.py
@@ -0,0 +1,16 @@
+# RUN: %python -m artiq.compiler.testbench.signature +diag %s >%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+def f():
+    delay_mu(2)
+
+def g():
+    delay_mu(2)
+
+x = f if True else g
+
+def h():
+    with parallel:
+        f()
+        # CHECK-L: ${LINE:+1}: fatal: it is not possible to interleave this function call within a 'with parallel:' statement because the compiler could not prove that the same function would always be called
+        x()