diff --git a/crates/runtime/src/externref.rs b/crates/runtime/src/externref.rs
index 9677b39afa..573dd4b24f 100644
--- a/crates/runtime/src/externref.rs
+++ b/crates/runtime/src/externref.rs
@@ -463,7 +463,7 @@ impl Deref for VMExternRef {
 ///
 /// We use this so that we can morally put `VMExternRef`s inside of `HashSet`s
 /// even though they don't implement `Eq` and `Hash` to avoid foot guns.
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 struct VMExternRefWithTraits(VMExternRef);
 
 impl Hash for VMExternRefWithTraits {
@@ -940,7 +940,9 @@ pub unsafe fn gc(
                         debug_assert!(
                             r.is_null() || activations_table_set.contains(&r),
                             "every on-stack externref inside a Wasm frame should \
-                            have an entry in the VMExternRefActivationsTable"
+                            have an entry in the VMExternRefActivationsTable; \
+                            {:?} is not in the table",
+                            r
                         );
                         if let Some(r) = NonNull::new(r) {
                             VMExternRefActivationsTable::insert_precise_stack_root(
diff --git a/crates/runtime/src/libcalls.rs b/crates/runtime/src/libcalls.rs
index 2544e16739..841dfaa1a2 100644
--- a/crates/runtime/src/libcalls.rs
+++ b/crates/runtime/src/libcalls.rs
@@ -402,6 +402,13 @@ pub unsafe extern "C" fn wasmtime_activations_table_insert_with_gc(
     let externref = VMExternRef::clone_from_raw(externref);
     let instance = (*vmctx).instance();
     let (activations_table, module_info_lookup) = (*instance.store()).externref_activations_table();
+
+    // Invariant: all `externref`s on the stack have an entry in the activations
+    // table. So we need to ensure that this `externref` is in the table
+    // *before* we GC, even though `insert_with_gc` will ensure that it is in
+    // the table *after* the GC.
+    activations_table.insert_without_gc(externref.clone());
+
     activations_table.insert_with_gc(externref, module_info_lookup);
 }