diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 24f001b43..981f49e0c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -19,6 +19,10 @@ jobs:
             numpy: "numpy"
             uncertainties: "uncertainties"
             extras: "sparse xarray netCDF4 dask[complete]==2024.5.1 graphviz babel==2.8 mip>=1.13"
+          - python-version: "3.10"
+            numpy: "numpy==1.26.1"
+            uncertainties: null
+            extras: "babel==2.15 matplotlib==3.9.0"
     runs-on: ubuntu-latest
 
     env:
diff --git a/pint/delegates/formatter/_compound_unit_helpers.py b/pint/delegates/formatter/_compound_unit_helpers.py
index 5757b46fa..06a8ac2d3 100644
--- a/pint/delegates/formatter/_compound_unit_helpers.py
+++ b/pint/delegates/formatter/_compound_unit_helpers.py
@@ -82,11 +82,19 @@ def localize_per(
     locale = babel_parse(locale)
 
     patterns = locale._data["compound_unit_patterns"].get("per", None)
+    if patterns is None:
+        return default or "{}/{}"
 
+    patterns = patterns.get(length, None)
     if patterns is None:
         return default or "{}/{}"
 
-    return patterns.get(length, default or "{}/{}")
+    # babel 2.8
+    if isinstance(patterns, str):
+        return patterns
+
+    # babe; 2.15
+    return patterns.get("compound", default or "{}/{}")
 
 
 @functools.lru_cache