diff --git a/llvm_mode/afl-clang-fast.c b/llvm_mode/afl-clang-fast.c
index 6a6414ad..1f3463eb 100644
--- a/llvm_mode/afl-clang-fast.c
+++ b/llvm_mode/afl-clang-fast.c
@@ -716,6 +716,9 @@ int main(int argc, char **argv, char **envp) {
     }
 
   }
+  
+  if (instrument_opt_mode && lto_mode)
+    FATAL("CTX and NGRAM can not be used in LTO mode (and would make LTO useless)");
 
   if (!instrument_opt_mode) {