diff --git a/changelog.d/ss-backend-preprocess-situation-hook.fixed.md b/changelog.d/ss-backend-preprocess-situation-hook.fixed.md
new file mode 100644
index 00000000000..4f65afe29fa
--- /dev/null
+++ b/changelog.d/ss-backend-preprocess-situation-hook.fixed.md
@@ -0,0 +1 @@
+Give the Social Security state test backend's stand-in system the situation preprocessing hook that policyengine-core's builder calls.
diff --git a/policyengine_us/tests/core/_social_security_state_backend.py b/policyengine_us/tests/core/_social_security_state_backend.py
index 7b11f34d019..5a90ed0d99d 100644
--- a/policyengine_us/tests/core/_social_security_state_backend.py
+++ b/policyengine_us/tests/core/_social_security_state_backend.py
@@ -266,6 +266,12 @@ class System:
             data_modified = False
             auto_carry_over_input_variables = True
 
+            def preprocess_situation(self, situation, default_period=None):
+                # Core's build_from_entities calls this country hook before
+                # building a situation (policyengine-core PR 600); the
+                # fixture's one-person situation needs no groups filled in.
+                return situation
+
             def __init__(self):
                 self.cache_blacklist = list(cache_blacklist)
                 self.variables = {