diff --git a/builtin/game/register.lua b/builtin/game/register.lua
index d111e5c27..51492e928 100644
--- a/builtin/game/register.lua
+++ b/builtin/game/register.lua
@@ -79,6 +79,7 @@ end
 
 function core.register_abm(spec)
 	-- Add to core.registered_abms
+	assert(type(spec.action) == "function", "Required field 'action' of type function")
 	core.registered_abms[#core.registered_abms + 1] = spec
 	spec.mod_origin = core.get_current_modname() or "??"
 end
@@ -86,6 +87,7 @@ end
 function core.register_lbm(spec)
 	-- Add to core.registered_lbms
 	check_modname_prefix(spec.name)
+	assert(type(spec.action) == "function", "Required field 'action' of type function")
 	core.registered_lbms[#core.registered_lbms + 1] = spec
 	spec.mod_origin = core.get_current_modname() or "??"
 end
diff --git a/src/script/cpp_api/s_env.cpp b/src/script/cpp_api/s_env.cpp
index f8cef98b7..ab3b5fe46 100644
--- a/src/script/cpp_api/s_env.cpp
+++ b/src/script/cpp_api/s_env.cpp
@@ -151,6 +151,10 @@ void ScriptApiEnv::initializeEnvironment(ServerEnvironment *env)
 		bool simple_catch_up = true;
 		getboolfield(L, current_abm, "catch_up", simple_catch_up);
 
+		lua_getfield(L, current_abm, "action");
+		luaL_checktype(L, current_abm + 1, LUA_TFUNCTION);
+		lua_pop(L, 1);
+
 		LuaABM *abm = new LuaABM(L, id, trigger_contents, required_neighbors,
 			trigger_interval, trigger_chance, simple_catch_up);
 
@@ -200,6 +204,10 @@ void ScriptApiEnv::initializeEnvironment(ServerEnvironment *env)
 		bool run_at_every_load = getboolfield_default(L, current_lbm,
 			"run_at_every_load", false);
 
+		lua_getfield(L, current_lbm, "action");
+		luaL_checktype(L, current_lbm + 1, LUA_TFUNCTION);
+		lua_pop(L, 1);
+
 		LuaLBM *lbm = new LuaLBM(L, id, trigger_contents, name,
 			run_at_every_load);