diff --git a/roottest/root/tree/index/CMakeLists.txt b/roottest/root/tree/index/CMakeLists.txt
index 31901e0e37505..33dede9115064 100644
--- a/roottest/root/tree/index/CMakeLists.txt
+++ b/roottest/root/tree/index/CMakeLists.txt
@@ -15,6 +15,10 @@ ROOTTEST_ADD_TEST(index64
                   MACRO runindex64.C
                   OUTREF index64.ref)
 
+ROOTTEST_ADD_TEST(indexl64
+                  MACRO runindexl64.C
+                  OUTREF indexl64.ref)
+
 ROOTTEST_ADD_TEST(varsizearr
                   MACRO runvarsizearr.C
                   OUTREF varsizearr.ref)
diff --git a/roottest/root/tree/index/runindexl64.C b/roottest/root/tree/index/runindexl64.C
index 9617fde08fbce..b859e3cf1892c 100644
--- a/roottest/root/tree/index/runindexl64.C
+++ b/roottest/root/tree/index/runindexl64.C
@@ -5,9 +5,8 @@
 bool test(TTree *);
 
 const char *fname = "indexl64.root";
-https : // github.com/root-project/root/pull/19561
-        const Long64_t bigval =
-           0x0FFFFFFFFFFFFFFF; // here we skip long double, so we can go higher than with runindex64.C
+// https://github.com/root-project/root/pull/19561
+const Long64_t bigval = 0x0FFFFFFFFFFFFFFF; // here we skip long double, so we can go higher than with runindex64.C
 
 int runindexl64()
 {
diff --git a/tree/treeplayer/src/TTreeIndex.cxx b/tree/treeplayer/src/TTreeIndex.cxx
index 7058671657afb..027cd296e2655 100644
--- a/tree/treeplayer/src/TTreeIndex.cxx
+++ b/tree/treeplayer/src/TTreeIndex.cxx
@@ -189,7 +189,7 @@ TTreeIndex::TTreeIndex(const TTree *T, const char *majorname, const char *minorn
          Error("TTreeIndex", "In tree entry %lld, Ndata in formula is zero for both '%s' and '%s'", i,
                fMajorName.Data(), fMinorName.Data());
       }
-      auto GetAndRangeCheck = [this](bool isMajor, Long64_t entry) {
+      auto GetAndRangeCheck = [this](bool isMajor, Long64_t entry) -> Long64_t {
          LongDouble_t ret = (isMajor ? fMajorFormula : fMinorFormula)->EvalInstance<LongDouble_t>();
          // Check whether the value (vs significant bits) of ldRet can represent
          // the full precision of the returned value. If we return 10^60, the
@@ -206,11 +206,16 @@ TTreeIndex::TTreeIndex(const TTree *T, const char *majorname, const char *minorn
                     "In tree entry %lld, %s value %s=%Lf possibly out of range for internal `long double`", entry,
                     isMajor ? "major" : "minor", isMajor ? fMajorName.Data() : fMinorName.Data(), ret);
          }
-         return ret;
+         return static_cast<Long64_t>(ret); // static_cast necessary for use in ternary expression below
       };
       auto GetLong64 = [this](bool isMajor) {
          return (isMajor ? fMajorFormula : fMinorFormula)->EvalInstance<Long64_t>();
       };
+      // Note: in ternary expression ensure both branches have same return data type Long64_t.
+      // otherwise, with LongDouble_t, the expression is implicitly cast to their
+      // common type `long double` regardless of the branch taken. On platforms where
+      // `sizeof(long double) == sizeof(double)` (macOS arm64, Windows) this silently
+      // rounds the exact Long64_t value, defeating the purpose of the long64 flag.
       tmp_major[i] = long64major ? GetLong64(true) : GetAndRangeCheck(true, i);
       tmp_minor[i] = long64minor ? GetLong64(false) : GetAndRangeCheck(false, i);
    }