diff --git a/async.c b/async.c
index a0dacae..bb029bd 100644
--- a/async.c
+++ b/async.c
@@ -717,14 +717,37 @@ PHP_FUNCTION(Async_runtime_stats)
 {
 	ZEND_PARSE_PARAMETERS_NONE();
 
+	/*
+	 * Before the scheduler is launched (e.g. runtime_stats() called at the top
+	 * level, outside any coroutine), the queue buffers are not yet allocated.
+	 * Report a zeroed snapshot rather than reading uninitialized state; the pool
+	 * minimum and fiber stack size are static and still meaningful.
+	 */
+	if (ZEND_ASYNC_SCHEDULER == NULL) {
+		array_init(return_value);
+		add_assoc_long(return_value, "coroutines_total",         0);
+		add_assoc_long(return_value, "coroutines_active",        0);
+		add_assoc_long(return_value, "microtasks_queue",         0);
+		add_assoc_long(return_value, "coroutine_queue",          0);
+		add_assoc_long(return_value, "resumed_queue",            0);
+		add_assoc_long(return_value, "fiber_pool_count",         0);
+		add_assoc_long(return_value, "fiber_pool_capacity",      0);
+		add_assoc_long(return_value, "fiber_pool_min",           (zend_long) ASYNC_FIBER_POOL_SIZE);
+		add_assoc_long(return_value, "fiber_stack_size",         (zend_long) EG(fiber_stack_size));
+		add_assoc_long(return_value, "fiber_pool_virtual_bytes", 0);
+		return;
+	}
+
 	const circular_buffer_t *const pool       = &ASYNC_G(fiber_context_pool);
 	const circular_buffer_t *const microtasks = &ASYNC_G(microtasks);
 	const circular_buffer_t *const coro_queue = &ASYNC_G(coroutine_queue);
 	const circular_buffer_t *const resumed_q  = &ASYNC_G(resumed_coroutines);
 
-	const size_t pool_count      = circular_buffer_count(pool);
-	/* circular_buffer_capacity does capacity-1, underflows to SIZE_MAX
-	 * pre-init. Clamp. */
+	/* Both reads guard against capacity 0: before the scheduler allocates these
+	 * buffers, circular_buffer_capacity() would underflow (capacity-1 = SIZE_MAX)
+	 * and circular_buffer_count() would read uninitialized head/tail. Clamp to 0. */
+	const size_t pool_count      = pool->capacity > 0
+	                                 ? circular_buffer_count(pool) : 0;
 	const size_t pool_capacity   = pool->capacity > 0
 	                                 ? circular_buffer_capacity(pool) : 0;
 	const size_t stack_size      = EG(fiber_stack_size);
@@ -737,11 +760,11 @@ PHP_FUNCTION(Async_runtime_stats)
 	               (zend_long) ZEND_ASYNC_ACTIVE_COROUTINE_COUNT);
 
 	add_assoc_long(return_value, "microtasks_queue",
-	               (zend_long) circular_buffer_count(microtasks));
+	               (zend_long) (microtasks->capacity > 0 ? circular_buffer_count(microtasks) : 0));
 	add_assoc_long(return_value, "coroutine_queue",
-	               (zend_long) circular_buffer_count(coro_queue));
+	               (zend_long) (coro_queue->capacity > 0 ? circular_buffer_count(coro_queue) : 0));
 	add_assoc_long(return_value, "resumed_queue",
-	               (zend_long) circular_buffer_count(resumed_q));
+	               (zend_long) (resumed_q->capacity > 0 ? circular_buffer_count(resumed_q) : 0));
 
 	add_assoc_long(return_value, "fiber_pool_count",    (zend_long) pool_count);
 	add_assoc_long(return_value, "fiber_pool_capacity", (zend_long) pool_capacity);
diff --git a/internal/circular_buffer.c b/internal/circular_buffer.c
index cdbdb98..91ff28a 100644
--- a/internal/circular_buffer.c
+++ b/internal/circular_buffer.c
@@ -520,6 +520,16 @@ bool circular_buffer_is_full(const circular_buffer_t *buffer)
 size_t circular_buffer_count(const circular_buffer_t *buffer)
 {
 	ZEND_ASSERT(buffer != NULL && "Buffer cannot be NULL");
+
+	/*
+	 * A buffer that was never initialized (capacity 0 — e.g. a scheduler queue
+	 * read before the scheduler has allocated it) holds nothing. Return 0 rather
+	 * than asserting the head/tail bounds, where 0 < 0 would fail.
+	 */
+	if (buffer->capacity == 0) {
+		return 0;
+	}
+
 	ZEND_ASSERT(buffer->head < buffer->capacity && "Head index out of bounds");
 	ZEND_ASSERT(buffer->tail < buffer->capacity && "Tail index out of bounds");
 
diff --git a/tests/common/runtime_stats.phpt b/tests/common/runtime_stats.phpt
new file mode 100644
index 0000000..a059d16
--- /dev/null
+++ b/tests/common/runtime_stats.phpt
@@ -0,0 +1,42 @@
+--TEST--
+Async\runtime_stats(): safe to call before the scheduler starts (#164)
+--FILE--
+<?php
+
+use function Async\runtime_stats;
+use function Async\spawn;
+use function Async\await;
+
+// Called at the top level: before any spawn/await the scheduler has not
+// launched and the queue buffers are unallocated. This used to abort the
+// process (assert head < capacity, i.e. 0 < 0) on a debug build; it must now
+// return a zeroed snapshot with the full set of keys.
+$stats = runtime_stats();
+var_dump(is_array($stats));
+
+$keys = ['coroutines_total', 'coroutines_active', 'microtasks_queue',
+         'coroutine_queue', 'resumed_queue', 'fiber_pool_count',
+         'fiber_pool_capacity', 'fiber_pool_min', 'fiber_stack_size',
+         'fiber_pool_virtual_bytes'];
+var_dump(array_diff($keys, array_keys($stats)) === []);
+
+// Live counters are zero before the scheduler runs; static fields still present.
+var_dump($stats['coroutines_total'] === 0);
+var_dump($stats['fiber_pool_virtual_bytes'] === 0);
+var_dump(is_int($stats['fiber_pool_min']));
+
+// Inside a coroutine the scheduler is running, so live values are reported.
+await(spawn(function (): void {
+    var_dump(runtime_stats()['coroutines_total'] >= 1);
+}));
+
+echo "done\n";
+?>
+--EXPECT--
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+done