diff --git a/src/ir/effects.h b/src/ir/effects.h
index 25a108342bc..472c5a8af79 100644
--- a/src/ir/effects.h
+++ b/src/ir/effects.h
@@ -1044,10 +1044,8 @@ class EffectAnalyzer {
     }
     void visitRefGetDesc(RefGetDesc* curr) { trapOnNull(curr->ref); }
     void visitBrOn(BrOn* curr) {
-      if (trapOnNull(curr->desc)) {
-        return;
-      }
       parent.breakTargets.insert(curr->name);
+      trapOnNull(curr->desc);
     }
     void visitStructNew(StructNew* curr) { trapOnNull(curr->desc); }
     void visitStructGet(StructGet* curr) {
diff --git a/test/lit/passes/monomorphize-desc.wast b/test/lit/passes/monomorphize-desc.wast
new file mode 100644
index 00000000000..9523bc664a8
--- /dev/null
+++ b/test/lit/passes/monomorphize-desc.wast
@@ -0,0 +1,65 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
+
+;; RUN: foreach %s %t wasm-opt --monomorphize -all -S -o - | filecheck %s
+
+(module
+ (rec
+  ;; CHECK:      (rec
+  ;; CHECK-NEXT:  (type $A (descriptor $B) (struct))
+  (type $A (descriptor $B) (struct))
+  ;; CHECK:       (type $B (describes $A) (struct))
+  (type $B (describes $A) (struct))
+ )
+
+ ;; CHECK:      (type $2 (func (param i32)))
+
+ ;; CHECK:      (type $3 (func (result anyref)))
+
+ ;; CHECK:      (type $4 (func (param nullref)))
+
+ ;; CHECK:      (func $target (type $2) (param $0 i32)
+ ;; CHECK-NEXT:  (nop)
+ ;; CHECK-NEXT: )
+ (func $target (param i32)
+ )
+
+ ;; CHECK:      (func $test (type $3) (result anyref)
+ ;; CHECK-NEXT:  (block $block (result (ref $A))
+ ;; CHECK-NEXT:   (call $target_2
+ ;; CHECK-NEXT:    (block ;; (replaces unreachable BrOn we can't emit)
+ ;; CHECK-NEXT:     (drop
+ ;; CHECK-NEXT:      (ref.null none)
+ ;; CHECK-NEXT:     )
+ ;; CHECK-NEXT:     (drop
+ ;; CHECK-NEXT:      (ref.null none)
+ ;; CHECK-NEXT:     )
+ ;; CHECK-NEXT:     (unreachable)
+ ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:   (unreachable)
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $test (result anyref)
+  ;; We can monomorphize the call to $target, pushing the ref.is_null into
+  ;; $target_2. The parameter we send there is now nullref, so everything there
+  ;; gets optimized away. In this function, after refinalizing we end up with
+  ;; an unprintable br_on (we don't have defined types for the null inputs, and
+  ;; in fact the br_on is never taken). While doing so, we should not hit an
+  ;; internal error.
+  (block $block (result (ref $A))
+   (call $target
+    (ref.is_null
+     (br_on_cast_desc_eq $block anyref (ref $A)
+      (ref.null none)
+      (ref.null none)
+     )
+    )
+   )
+   (unreachable)
+  )
+ )
+)
+
+;; CHECK:      (func $target_2 (type $4) (param $0 nullref)
+;; CHECK-NEXT:  (nop)
+;; CHECK-NEXT: )