diff --git a/core/metacling/src/TCling.cxx b/core/metacling/src/TCling.cxx
index 6a7ceb7f70cb2..0b99545b6d58a 100644
--- a/core/metacling/src/TCling.cxx
+++ b/core/metacling/src/TCling.cxx
@@ -1105,7 +1105,10 @@ static bool LoadModule(const std::string &ModuleName, cling::Interpreter &interp
    cling::Interpreter::PushTransactionRAII RAII(&interp);
    if (clang::Module *M = moduleMap.findModule(ModuleName)) {
       clang::IdentifierInfo *II = PP.getIdentifierInfo(M->Name);
-      return !CI.getSema().ActOnModuleImport(ValidLoc, ValidLoc, std::make_pair(II, ValidLoc)).isInvalid();
+      bool Result = !CI.getSema().ActOnModuleImport(ValidLoc, ValidLoc, std::make_pair(II, ValidLoc)).isInvalid();
+      // Also make the module visible in the preprocessor to export its macros.
+      PP.makeModuleVisible(M, ValidLoc);
+      return Result;
    }
    return false;
 }
@@ -1138,6 +1141,17 @@ static void LoadCoreModules(cling::Interpreter &interp)
 
    if (!LoadModule(moduleMap.findModule("RIO")->Name, interp))
       Error("TCling::LoadCoreModules", "Cannot load module RIO");
+
+   // Check that the gROOT macro was exported by any core module.
+   assert(interp.getMacro("gROOT") && "Couldn't load gROOT macro?");
+
+   // 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
+   interp.declare("#ifdef I\n #undef I\n #endif\n");
 }
 
 static bool FileExists(const char *file)