#51826_Fixed bug when np.nan is used as index value with .reindex on …

pandas/_libs/intervaltree.pxi.in

@@ -391,6 +391,10 @@ cdef class {{dtype_title}}Closed{{closed_title}}IntervalNode(IntervalNode):
         """Recursively query this node and its sub-nodes for intervals that
         overlap with the query point.
         """
+        # GH 51826: ensures nan is handled properly during reindexing
+        if np.isnan(point):
+            return
+
         cdef:
             int64_t[:] indices
             {{dtype}}_t[:] values

pandas/tests/indexing/interval/test_interval.py

@@ -172,3 +172,20 @@ def test_mi_intervalindex_slicing_with_scalar(self):
         )
         expected = Series([1, 6, 2, 8, 7], index=expected_index, name="value")
         tm.assert_series_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "base, expected_result",
+        [
+            (10, Series([np.nan, 0], index=[np.nan, 1.0], dtype=np.float64)),
+            (100, Series([np.nan, 0], index=[np.nan, 1.0], dtype=np.float64)),
+            (101, Series([np.nan, 0], index=[np.nan, 1.0], dtype=np.float64)),
+            (1010, Series([np.nan, 0], index=[np.nan, 1.0], dtype=np.float64)),
+        ],
+    )
+    def test_interval_index_reindex_behavior(self, base, expected_result):
+        # GH 51826
+        left = np.arange(base, dtype=np.int32)
+        right = np.arange(1, base + 1, dtype=np.int32)
+        d = Series(range(base), index=IntervalIndex.from_arrays(left, right))
+        result = d.reindex(index=[np.nan, 1.0])
+        tm.assert_series_equal(result, expected_result)