diff --git a/src/modules/graphics/wrap_Graphics.cpp b/src/modules/graphics/wrap_Graphics.cpp
index a423453a4..beab3df90 100644
--- a/src/modules/graphics/wrap_Graphics.cpp
+++ b/src/modules/graphics/wrap_Graphics.cpp
@@ -2395,6 +2395,8 @@ int w_readbackTexture(lua_State *L)
 	int slice = 0;
 	if (t->getTextureType() != TEXTURE_2D)
 		slice = (int) luaL_checkinteger(L, 2) - 1;
+	else if (!lua_isnoneornil(L, 2) && lua_type(L, 2) != LUA_TNUMBER)
+		luaL_argerror(L, 2, "number or nil expected");
 
 	int mipmap = (int) luaL_optinteger(L, 3, 1) - 1;
 
@@ -2433,6 +2435,8 @@ int w_readbackTextureAsync(lua_State *L)
 	int slice = 0;
 	if (t->getTextureType() != TEXTURE_2D)
 		slice = (int) luaL_checkinteger(L, 2) - 1;
+	else if (!lua_isnoneornil(L, 2) && lua_type(L, 2) != LUA_TNUMBER)
+		luaL_argerror(L, 2, "number or nil expected");
 
 	int mipmap = (int) luaL_optinteger(L, 3, 1) - 1;