diff --git a/CHANGELOG.md b/CHANGELOG.md
index 87e2d13..8e66a6e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,7 +10,10 @@ follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 - Contracts may set `mode_type` — `replace` (the default), `append` or
   `upsert` — and, for `upsert` only, `primary_key`: a non-empty list of output
   columns, each spelled exactly as in `output_columns` and declared
-  `nullable: false`, names unique case-insensitively.
+  `nullable: false`, names unique case-insensitively, and none a
+  floating-point (`DOUBLE PRECISION`) column — the warehouse collapses `0.0`
+  and `-0.0`, so a float key is rejected at validation rather than failing at
+  write time.
   `python-csv` nodes allow only `replace`. `continuo-runtime validate`, `merge`
   and `run` enforce these rules; the harness's write path for the new modes
   follows in a later entry.
diff --git a/continuo_python_runtime/contract/loader.py b/continuo_python_runtime/contract/loader.py
index c5f8927..b37fd6b 100644
--- a/continuo_python_runtime/contract/loader.py
+++ b/continuo_python_runtime/contract/loader.py
@@ -155,7 +155,10 @@ def _validate_primary_key(
     output column (postgres quotes identifiers, so ``Rate_Date`` is not
     ``rate_date``) declared ``nullable: false``, because a NULL key never
     matches an existing row. Repeats are refused case-insensitively too, so a
-    key cannot name two columns that differ only in case.
+    key cannot name two columns that differ only in case. A floating-point
+    column is refused as well: the warehouse treats ``0.0`` and ``-0.0`` as
+    one key, so ``ensure_unique_key`` would reject the rows at write time --
+    failing here instead keeps an unwritable upsert out of the contract.
     """
     if mode_type != "upsert":
         if "primary_key" in raw:
@@ -194,6 +197,11 @@ def _validate_primary_key(
             raise ContractError(
                 f"{label}: 'primary_key' column {name!r} must be declared nullable: false"
             )
+        if parse_sql_type(column.type).base == "DOUBLE_PRECISION":
+            raise ContractError(
+                f"{label}: 'primary_key' column {name!r} is floating-point "
+                f"({column.type}); a floating-point column cannot be a primary key"
+            )
     return tuple(raw_key)
 
 
diff --git a/tests/contract/test_loader.py b/tests/contract/test_loader.py
index 94475ef..675461f 100644
--- a/tests/contract/test_loader.py
+++ b/tests/contract/test_loader.py
@@ -853,6 +853,16 @@ def test_primary_key_column_must_be_not_null():
         parse_node(_upsert_entry(primary_key=["rate_date", "rate"]), "c.yml")
 
 
+def test_primary_key_column_cannot_be_floating_point():
+    """ensure_unique_key rejects float keys at write time; refuse them at validation."""
+    entry = _upsert_entry(primary_key=["rate_date", "score"])
+    entry["output_columns"].append({"name": "score", "type": "DOUBLE PRECISION", "nullable": False})
+    with pytest.raises(
+        ContractError, match="'primary_key' column 'score' is floating-point"
+    ):
+        parse_node(entry, "c.yml")
+
+
 def test_primary_key_names_must_match_output_columns_exactly():
     """Review Focus 2: postgres quotes identifiers, so Rate_Date is not rate_date."""
     with pytest.raises(