diff --git a/src/libexpr/nix_api_expr.cc b/src/libexpr/nix_api_expr.cc
index 84d55ec13..880030380 100644
--- a/src/libexpr/nix_api_expr.cc
+++ b/src/libexpr/nix_api_expr.cc
@@ -110,33 +110,50 @@ std::unordered_map<
 
 std::mutex nix_refcount_lock;
 
-void nix_gc_incref(const void *p) {
-  std::scoped_lock lock(nix_refcount_lock);
-  auto f = nix_refcounts.find(p);
-  if (f != nix_refcounts.end()) {
-    f->second++;
-  } else {
-    nix_refcounts[p] = 1;
+nix_err nix_gc_incref(nix_c_context *context, const void *p) {
+  if (context)
+    context->last_err_code = NIX_OK;
+  try {
+    std::scoped_lock lock(nix_refcount_lock);
+    auto f = nix_refcounts.find(p);
+    if (f != nix_refcounts.end()) {
+      f->second++;
+    } else {
+      nix_refcounts[p] = 1;
+    }
   }
+  NIXC_CATCH_ERRS
 }
 
-void nix_gc_decref(const void *p) {
-  std::scoped_lock lock(nix_refcount_lock);
-  auto f = nix_refcounts.find(p);
-  if (f != nix_refcounts.end()) {
-    if (f->second == 1)
-      nix_refcounts.erase(f);
-    else
-      f->second--;
+nix_err nix_gc_decref(nix_c_context *context, const void *p) {
+
+  if (context)
+    context->last_err_code = NIX_OK;
+  try {
+    std::scoped_lock lock(nix_refcount_lock);
+    auto f = nix_refcounts.find(p);
+    if (f != nix_refcounts.end()) {
+      if (--f->second == 0)
+        nix_refcounts.erase(f);
+    } else
+      throw std::runtime_error("nix_gc_decref: object was not referenced");
   }
-  // todo: else { throw? }
+  NIXC_CATCH_ERRS
 }
 
 void nix_gc_now() { GC_gcollect(); }
 
 #else
-void nix_gc_incref(const void *) {}
-void nix_gc_decref(const void *) {}
+void nix_gc_incref(nix_c_context *context, const void *) {
+  if (context)
+    context->last_err_code = NIX_OK;
+  return NIX_OK;
+}
+void nix_gc_decref(nix_c_context *context, const void *) {
+  if (context)
+    context->last_err_code = NIX_OK;
+  return NIX_OK;
+}
 void nix_gc_now() {}
 #endif
 
diff --git a/src/libexpr/nix_api_expr.h b/src/libexpr/nix_api_expr.h
index 9efb3dde1..c56ef89bb 100644
--- a/src/libexpr/nix_api_expr.h
+++ b/src/libexpr/nix_api_expr.h
@@ -120,13 +120,13 @@ void nix_state_free(State *state);
  *
  * @param[in] object The object to keep alive
  */
-void nix_gc_incref(const void *);
+nix_err nix_gc_incref(nix_c_context *, const void *);
 /**
  * @brief Decrease the GC refcount
  *
  * @param[in] object The object to stop referencing
  */
-void nix_gc_decref(const void *);
+nix_err nix_gc_decref(nix_c_context *, const void *);
 
 /**
  * @brief Trigger the garbage collector manually
diff --git a/src/libexpr/nix_api_external.cc b/src/libexpr/nix_api_external.cc
index d72adee80..a927a4037 100644
--- a/src/libexpr/nix_api_external.cc
+++ b/src/libexpr/nix_api_external.cc
@@ -178,7 +178,7 @@ ExternalValue *nix_create_external_value(nix_c_context *context,
         (GC)
 #endif
             NixCExternalValue(*desc, v);
-    nix_gc_incref(ret);
+    nix_gc_incref(nullptr, ret);
     return (ExternalValue *)ret;
   }
   NIXC_CATCH_ERRS_NULL
diff --git a/src/libexpr/nix_api_value.cc b/src/libexpr/nix_api_value.cc
index f34907ef1..6e02b3310 100644
--- a/src/libexpr/nix_api_value.cc
+++ b/src/libexpr/nix_api_value.cc
@@ -49,7 +49,7 @@ PrimOp *nix_alloc_primop(nix_c_context *context, PrimOpFun fun, int arity,
     if (args)
       for (size_t i = 0; args[i]; i++)
         p->args.emplace_back(*args);
-    nix_gc_incref(p);
+    nix_gc_incref(nullptr, p);
     return (PrimOp *)p;
   }
   NIXC_CATCH_ERRS_NULL
@@ -60,7 +60,7 @@ Value *nix_alloc_value(nix_c_context *context, State *state) {
     context->last_err_code = NIX_OK;
   try {
     Value *res = state->state.allocValue();
-    nix_gc_incref(res);
+    nix_gc_incref(nullptr, res);
     return res;
   }
   NIXC_CATCH_ERRS_NULL
@@ -208,7 +208,7 @@ Value *nix_get_list_byidx(nix_c_context *context, const Value *value,
     auto &v = check_value_not_null(value);
     assert(v.type() == nix::nList);
     auto *p = v.listElems()[ix];
-    nix_gc_incref(p);
+    nix_gc_incref(nullptr, p);
     return (Value *)p;
   }
   NIXC_CATCH_ERRS_NULL
@@ -224,7 +224,7 @@ Value *nix_get_attr_byname(nix_c_context *context, const Value *value,
     nix::Symbol s = state->state.symbols.create(name);
     auto attr = v.attrs->get(s);
     if (attr) {
-      nix_gc_incref(attr->value);
+      nix_gc_incref(nullptr, attr->value);
       return attr->value;
     }
     nix_set_err_msg(context, NIX_ERR_KEY, "missing attribute");
@@ -257,7 +257,7 @@ Value *nix_get_attr_byidx(nix_c_context *context, const Value *value,
     auto &v = check_value_not_null(value);
     const nix::Attr &a = (*v.attrs)[i];
     *name = ((const std::string &)(state->state.symbols[a.name])).c_str();
-    nix_gc_incref(a.value);
+    nix_gc_incref(nullptr, a.value);
     return a.value;
   }
   NIXC_CATCH_ERRS_NULL