...

src/CLI/Program.cs

@@ -89,6 +89,10 @@ foreach (var compilationUnit in compilationUnits)
     File.WriteAllText(ssaPath, ssa);
     
     var asm = await QBE.Invoke(ssa);
+    if (asm == null)
+    {
+        return 1;
+    }
 
     var asmPath = Path.ChangeExtension(outputPath, "s");
     await File.WriteAllTextAsync(asmPath, asm);