diff --git a/CHANGELOG.md b/CHANGELOG.md
index 318cdea10..d234e683b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed `JobListParams.Where` writing into the first `NamedArgs` it's given when merging multiple sets of named arguments, which could leak arguments into a map the caller reused for another query. It also no longer panics when the first set is `nil`. [PR #1575](https://github.com/riverqueue/river/pull/1575).
+
 ## [0.51.1] - 2026-11-05
 
 ### Fixed
diff --git a/job_list_params.go b/job_list_params.go
index bddd1be0c..a42bbe9c9 100644
--- a/job_list_params.go
+++ b/job_list_params.go
@@ -576,14 +576,11 @@ func (p *JobListParams) Where(sql string, namedArgsMany ...NamedArgs) *JobListPa
 	paramsCopy := p.copy()
 
 	var allNamedArgs NamedArgs
-	if len(namedArgsMany) > 0 {
-		for i, namedArgs := range namedArgsMany {
-			if i == 0 {
-				allNamedArgs = namedArgs
-			} else {
-				maps.Copy(allNamedArgs, namedArgs)
-			}
+	for _, namedArgs := range namedArgsMany {
+		if allNamedArgs == nil {
+			allNamedArgs = make(NamedArgs, len(namedArgs))
 		}
+		maps.Copy(allNamedArgs, namedArgs)
 	}
 
 	paramsCopy.where = append(paramsCopy.where, dblist.WherePredicate{NamedArgs: allNamedArgs, SQL: sql})
diff --git a/job_list_params_test.go b/job_list_params_test.go
index e964a5975..0e741ed39 100644
--- a/job_list_params_test.go
+++ b/job_list_params_test.go
@@ -636,3 +636,28 @@ func Test_JobListParams_toDBParamsWithoutFinalizedIndex(t *testing.T) {
 		})
 	}
 }
+
+func TestJobListParams_Where(t *testing.T) {
+	t.Parallel()
+
+	t.Run("MergesNamedArgsWithoutMutatingThem", func(t *testing.T) {
+		t.Parallel()
+
+		var (
+			namedArgs1 = NamedArgs{"a": 1, "b": 2}
+			namedArgs2 = NamedArgs{"b": 3}
+		)
+
+		params := NewJobListParams().Where("a = @a AND b = @b", namedArgs1, namedArgs2)
+		require.Equal(t, map[string]any{"a": 1, "b": 3}, params.where[0].NamedArgs)
+		require.Equal(t, NamedArgs{"a": 1, "b": 2}, namedArgs1)
+		require.Equal(t, NamedArgs{"b": 3}, namedArgs2)
+	})
+
+	t.Run("NilFirstNamedArgs", func(t *testing.T) {
+		t.Parallel()
+
+		params := NewJobListParams().Where("a = @a", nil, NamedArgs{"a": 1})
+		require.Equal(t, map[string]any{"a": 1}, params.where[0].NamedArgs)
+	})
+}