diff --git a/src/libclang/preprocessor.cpp b/src/libclang/preprocessor.cpp
index b5a641c..935b871 100644
--- a/src/libclang/preprocessor.cpp
+++ b/src/libclang/preprocessor.cpp
@@ -226,8 +226,8 @@ namespace
         // other flags
         for (const auto& flag : detail::libclang_compile_config_access::flags(c))
         {
-            DEBUG_ASSERT(flag.size() > 2u && flag[0] == '-', detail::assert_handler{},
-                         "that's an odd flag");
+            DEBUG_ASSERT(flag.size() >= 2u && flag[0] == '-', detail::assert_handler{},
+                         ("\"" + flag + "\" that's an odd flag").c_str());
             if (!macro_file_path || flag[1] != 'I')
             {
                 // only add this flag if it is not an include or we're not doing fast preprocessing