diff --git a/kernel/fstdata.cc b/kernel/fstdata.cc
index 0b62564f6..330c4d189 100644
--- a/kernel/fstdata.cc
+++ b/kernel/fstdata.cc
@@ -26,6 +26,8 @@ FstData::FstData(std::string filename) : ctx(nullptr)
 {
 	const std::vector<std::string> g_units = { "s", "ms", "us", "ns", "ps", "fs", "as", "zs" };
 	ctx = (fstReaderContext *)fstReaderOpen(filename.c_str());
+	if (!ctx)
+		log_error("Error opening '%s'\n", filename.c_str());
 	int scale = (int)fstReaderGetTimescale(ctx);	
 	timescale = pow(10.0, scale);
 	timescale_str = "";
diff --git a/passes/sat/sim.cc b/passes/sat/sim.cc
index b2425b531..0687e3e0f 100644
--- a/passes/sat/sim.cc
+++ b/passes/sat/sim.cc
@@ -961,6 +961,9 @@ struct SimWorker : SimShared
 		log_assert(top == nullptr);
 		fst = new FstData(sim_filename);
 
+		if (scope.empty())
+			log_error("Scope must be defined for co-simulation.\n");
+
 		top = new SimInstance(this, scope, topmod);
 
 		std::vector<fstHandle> fst_clock;