Index: uspace/lib/c/generic/async/client.c
===================================================================
--- uspace/lib/c/generic/async/client.c	(revision 58e7b26977afda549d7da683201907528ba22fd9)
+++ uspace/lib/c/generic/async/client.c	(revision be34d6f21a9f4b1100d4ecca3d63e78f609018f1)
@@ -751,5 +751,8 @@
 	fibril_mutex_lock(&async_sess_mutex);
 
-	assert(sess->exchanges == 0);
+	if (sess->exchanges > 0) {
+		fibril_mutex_unlock(&async_sess_mutex);
+		return EBUSY;
+	}
 
 	errno_t rc = async_hangup_internal(sess->phone);