diff --git a/include/fmt/core.h b/include/fmt/core.h
index 23d2565e..467fdb86 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -935,11 +935,11 @@ template <typename T> class buffer {
   /** Appends data to the end of the buffer. */
   template <typename U> void append(const U* begin, const U* end);
 
-  template <typename I> FMT_CONSTEXPR auto operator[](I index) -> T& {
+  template <typename Idx> FMT_CONSTEXPR auto operator[](Idx index) -> T& {
     return ptr_[index];
   }
-  template <typename I>
-  FMT_CONSTEXPR auto operator[](I index) const -> const T& {
+  template <typename Idx>
+  FMT_CONSTEXPR auto operator[](Idx index) const -> const T& {
     return ptr_[index];
   }
 };
diff --git a/test/core-test.cc b/test/core-test.cc
index 96cde1ed..c76dc161 100644
--- a/test/core-test.cc
+++ b/test/core-test.cc
@@ -9,7 +9,9 @@
 #include "test-assert.h"
 // clang-format on
 
+#define I 42  // simulate https://en.cppreference.com/w/c/numeric/complex/I
 #include "fmt/core.h"
+#undef I
 
 #include <algorithm>    // std::copy_n
 #include <climits>      // INT_MAX