diff --git a/src/loop.c b/src/loop.c
index 6053032c..df7be818 100644
--- a/src/loop.c
+++ b/src/loop.c
@@ -32,6 +32,11 @@ static const char *const luv_runmodes[] = {
 static int luv_run(lua_State* L) {
   int mode = luaL_checkoption(L, 1, "default", luv_runmodes);
   luv_ctx_t* ctx = luv_context(L);
+  if (ctx->mode != -1) {
+    lua_pushnil(L);
+    lua_pushstring(L, "loop already running");
+    return 2;
+  }
   ctx->mode = mode;
   int ret = uv_run(ctx->loop, (uv_run_mode)mode);
   ctx->mode = -1;