diff --git a/core/metacling/src/TCling.cxx b/core/metacling/src/TCling.cxx
index 5af6fe7d514eb..2dea465023cfe 100644
--- a/core/metacling/src/TCling.cxx
+++ b/core/metacling/src/TCling.cxx
@@ -1101,19 +1101,33 @@ static void LoadCoreModules(cling::Interpreter &interp)
 
    clang::HeaderSearch &headerSearch = CI.getPreprocessor().getHeaderSearchInfo();
    clang::ModuleMap &moduleMap = headerSearch.getModuleMap();
+   // List of core modules we can load, but it's ok if they are missing because
+   // the system doesn't have these modules.
+   std::vector<std::string> optionalCoreModuleNames = {"stl", "libc"};
+
    // List of core modules we need to load.
-   std::vector<std::string> neededCoreModuleNames = {"Core", "RIO"};
+   // FIXME: TreePlayer is here because the rootmap loading doesn't work with
+   // TDataFrame. This should be gone once we no longer rely on rootmap files
+   // for loading a module.
+   std::vector<std::string> neededCoreModuleNames = {"Core", "RIO", "TreePlayer"};
    std::vector<std::string> missingCoreModuleNames;
 
    std::vector<clang::Module *> coreModules;
 
+   // Lookup the optional core modules in the modulemap by name.
+   for (std::string moduleName : optionalCoreModuleNames) {
+      clang::Module *module = moduleMap.findModule(moduleName);
+      // Don't report an error here, the module is optional.
+      if (module)
+         coreModules.push_back(module);
+   }
    // Lookup the core modules in the modulemap by name.
    for (std::string moduleName : neededCoreModuleNames) {
       clang::Module *module = moduleMap.findModule(moduleName);
       if (module) {
          coreModules.push_back(module);
       } else {
-         // If we can't find a module, we record that to report it later.
+         // If we can't find a needed module, we record that to report it later.
          missingCoreModuleNames.push_back(moduleName);
       }
    }
@@ -1152,6 +1166,15 @@ static void LoadCoreModules(cling::Interpreter &interp)
    for (StringRef H : moduleHeaders) {
       declarations << "#include \"" << H.str() << "\"\n";
    }
+
+   // C99 decided that it's a very good idea to name a macro `I` (the letter I).
+   // This seems to screw up nearly all the template code out there as `I` is
+   // common template parameter name and iterator variable name.
+   // Let's follow the GCC recommendation and undefine `I` in case any of the
+   // core modules have defined it:
+   // https://www.gnu.org/software/libc/manual/html_node/Complex-Numbers.html
+   declarations << "#ifdef I\n #undef I\n #endif\n";
+
    auto result = interp.declare(declarations.str());
 
    if (result != cling::Interpreter::CompilationResult::kSuccess) {