diff --git a/backend/app/schemas/goal.py b/backend/app/schemas/goal.py
index 1efd717..1610631 100644
--- a/backend/app/schemas/goal.py
+++ b/backend/app/schemas/goal.py
@@ -1,7 +1,7 @@
 from datetime import date as date_
 from decimal import Decimal
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, field_validator
 
 
 class GoalCreate(BaseModel):
@@ -19,10 +19,20 @@ class GoalUpdate(BaseModel):
 class GoalContributionCreate(BaseModel):
     # Negative allowed — a withdrawal from the goal is still a contribution
     # to its running total, just in the other direction. Zero is pointless.
-    amount: Decimal = Field(ne=0)
+    amount: Decimal
     date: date_
     note: str | None = Field(default=None, max_length=200)
 
+    @field_validator("amount")
+    @classmethod
+    def _reject_zero(cls, value: Decimal) -> Decimal:
+        # Has to be a validator: pydantic has no "not equal" constraint, so
+        # the Field(ne=0) this replaces was parsed as an unknown keyword,
+        # stored as schema metadata, and never actually checked anything.
+        if value == 0:
+            raise ValueError("amount must not be zero")
+        return value
+
 
 class GoalRead(BaseModel):
     id: int
diff --git a/backend/tests/test_goals.py b/backend/tests/test_goals.py
new file mode 100644
index 0000000..98638b8
--- /dev/null
+++ b/backend/tests/test_goals.py
@@ -0,0 +1,34 @@
+"""Savings goals: contribution rules.
+
+The contribution log accepts negative amounts on purpose (taking money back
+out of a goal is still an entry in its running total), which is exactly why
+zero has to be rejected explicitly rather than by a `gt=0` constraint.
+"""
+from httpx import AsyncClient
+
+from tests.helpers import money
+
+
+async def _goal(client: AsyncClient) -> dict:
+    return (await client.post("/goals", json={"name": "Emergency fund", "target_amount": "1000"})).json()
+
+
+async def test_zero_contribution_is_rejected(client: AsyncClient):
+    goal = await _goal(client)
+
+    resp = await client.post(f"/goals/{goal['id']}/contributions", json={"amount": "0", "date": "2026-01-15"})
+
+    assert resp.status_code == 422
+    assert money((await client.get("/goals")).json()[0]["current_amount"]) == money(0)
+
+
+async def test_negative_contribution_is_allowed(client: AsyncClient):
+    """Withdrawing from a goal is a legitimate entry — the zero check must not
+    turn into a positive-only constraint."""
+    goal = await _goal(client)
+    await client.post(f"/goals/{goal['id']}/contributions", json={"amount": "250", "date": "2026-01-15"})
+
+    resp = await client.post(f"/goals/{goal['id']}/contributions", json={"amount": "-100", "date": "2026-02-01"})
+
+    assert resp.status_code == 201
+    assert money(resp.json()["current_amount"]) == money("150")