diff --git a/xarray/core/combine.py b/xarray/core/combine.py
index 167b5a93ebc..e35bb51e030 100644
--- a/xarray/core/combine.py
+++ b/xarray/core/combine.py
@@ -248,7 +248,7 @@ def _combine_1d(
                 join=join,
             )
         except ValueError as err:
-            if "Encountered unexpected variable" in str(err):
+            if "encountered unexpected variable" in str(err):
                 raise ValueError(
                     "These objects cannot be combined using only "
                     "xarray.combine_nested, instead either use "