diff --git a/docs/docs/guides/memory.md b/docs/docs/guides/memory.md
index 998c1d2d..0ae3b477 100644
--- a/docs/docs/guides/memory.md
+++ b/docs/docs/guides/memory.md
@@ -389,7 +389,7 @@ This tells you the join needed about 1 GB of temporary memory beyond what was al
 | Tool | What It Does | When to Use |
 |---|---|---|
 | `(.sys.mem 0)` | Returns heap allocation statistics | Monitor memory usage, detect leaks |
-| `(.sys.gc 0)` | Flushes caches, releases pages | Between heavy queries, before benchmarks |
+| `(.sys.gc)` | Flushes caches, releases pages | Between heavy queries, before benchmarks |
 | `(.sys.info 0)` | Shows system and runtime info | Check total RAM, CPU count, OS details |
 | `(timeit expr)` | Measures execution time of one expression | Benchmark a specific operation |
 | `:timeit` | Toggles profiling for all REPL expressions | Interactive performance exploration |
diff --git a/src/ops/system.c b/src/ops/system.c
index 17641564..c72e2da4 100644
--- a/src/ops/system.c
+++ b/src/ops/system.c
@@ -872,7 +872,8 @@ ray_t* ray_mem_ts_fn(ray_t** args, int64_t n) {
  * pages/pools.  Rayforce values are reference-counted, so this is allocator
  * GC rather than a tracing collector.  Variadic to allow `(.sys.gc)`. */
 ray_t* ray_gc_fn(ray_t** args, int64_t n) {
-    (void)args; (void)n;
+    (void)args;
+    if (n != 0) return ray_error("domain", ".sys.gc takes no arguments");
     ray_heap_gc();
     /* Same statement-boundary rule as the REPL: an explicit maintenance
      * call is also a chance to notice the process has gone quiet. */
diff --git a/test/rfl/system/reserved_namespace.rfl b/test/rfl/system/reserved_namespace.rfl
index fd0d7238..d8db459a 100644
--- a/test/rfl/system/reserved_namespace.rfl
+++ b/test/rfl/system/reserved_namespace.rfl
@@ -5,10 +5,10 @@
 ;; `'?` regardless of arity — so we exercise by calling, which is
 ;; what actually matters here.)
 ;; .sys.gc returns 0 on success — doubles as a binding-exists probe.
-;; Registered as variadic so both (.sys.gc) and (.sys.gc 0) work;
-;; other .sys.* info builtins follow the same convention.
+;; Registered as variadic so the zero-argument call remains valid while
+;; the implementation can reject unexpected arguments explicitly.
 (.sys.gc) -- 0
-(.sys.gc 0) -- 0
+(.sys.gc 0) !- domain
 (count (.sys.info)) -- 5
 (count (.sys.mem)) -- 16
 (count (.sys.build)) -- 2
@@ -116,7 +116,7 @@ internals !- name
 (del .sys.gc) !- reserve
 (del .os.getenv) !- reserve
 ;; Built-in still resolves after the blocked shadow attempts.
-(.sys.gc 0) -- 0
+(.sys.gc) -- 0
 (nil? .os.getenv) -- false
 ;; User-level dotted writes under a non-`.` root still work.
 (set myns.x 1)
diff --git a/test/rfl/system/system_branch_cov.rfl b/test/rfl/system/system_branch_cov.rfl
index 32adf1dd..a139b508 100644
--- a/test/rfl/system/system_branch_cov.rfl
+++ b/test/rfl/system/system_branch_cov.rfl
@@ -179,8 +179,8 @@
 ;; ray_gc_fn  (line 539)
 ;; ══════════════════════════════════════════════════════════════════════
 (.sys.gc) -- 0
-(.sys.gc 0) -- 0
-(.sys.gc "x") -- 0
+(.sys.gc 0) !- domain
+(.sys.gc "x") !- domain
 
 ;; ────────────── teardown ──────────────
 (.sys.exec "rm -rf /tmp/rfl_sys_bc_*")