Fixed unexpected np.nan value with reindex on pd.series with pd.Inter…

pandas/_libs/intervaltree.pxi.in

@@ -391,6 +391,11 @@ 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

@@ -1,6 +1,8 @@
 import numpy as np
 import pytest
 
+from pandas.compat import IS64
+
 import pandas as pd
 from pandas import (
     DataFrame,
@@ -172,3 +174,19 @@ 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.xfail(not IS64, reason="GH 23440")
+    @pytest.mark.parametrize(
+        "base",
+        [101, 1010],
+    )
+    def test_reindex_behavior_with_interval_index(self, base):
+        # GH 51826
+
+        ser = Series(
+            range(base),
+            index=IntervalIndex.from_arrays(range(base), range(1, base + 1)),
+        )
+        expected_result = Series([np.nan, 0], index=[np.nan, 1.0], dtype=float)
+        result = ser.reindex(index=[np.nan, 1.0])
+        tm.assert_series_equal(result, expected_result)