Fix get_compatible_units for dynamically add units/dimensions

Close #1725

2 files changed, 6 insertions, 1 deletions

diff --git a/pint/facets/plain/registry.py b/pint/facets/plain/registry.py
index 94cc5ff..b7ca362 100644
--- a/pint/facets/plain/registry.py
+++ b/pint/facets/plain/registry.py
@@ -854,7 +854,7 @@ class PlainRegistry(metaclass=RegistryMeta):
             return frozenset()
 
         src_dim = self._get_dimensionality(input_units)
-        return self._cache.dimensional_equivalents[src_dim]
+        return self._cache.dimensional_equivalents.setdefault(src_dim, set())
 
     # TODO: remove context from here
     def is_compatible_with(
diff --git a/pint/testsuite/test_issues.py b/pint/testsuite/test_issues.py
index 8f16f81..6dd8f85 100644
--- a/pint/testsuite/test_issues.py
+++ b/pint/testsuite/test_issues.py
@@ -1073,3 +1073,8 @@ def test_issue1631():
     q = 2 * ureg.meter
     assert isinstance(q, ureg.Quantity)
     assert isinstance(q, pint.Quantity)
+
+
+def test_issue1725(registry_empty):
+    registry_empty.define("dollar = [currency]")
+    assert registry_empty.get_compatible_units("dollar") == set()