diff --git a/pkg/restheadspec/handler.go b/pkg/restheadspec/handler.go
index 9beb02c..3600e3d 100644
--- a/pkg/restheadspec/handler.go
+++ b/pkg/restheadspec/handler.go
@@ -241,9 +241,11 @@ func (h *Handler) Handle(w common.ResponseWriter, r common.Request, params map[s
 			h.sendError(w, http.StatusBadRequest, "invalid_request", "Invalid request body", err)
 			return
 		}
-		validId, _ := strconv.ParseInt(id, 10, 64)
+		// A URL id is valid when it is a positive integer or any non-numeric
+		// string (string primary keys); "", "0" and negatives mean no id.
+		validId, parseErr := strconv.ParseInt(id, 10, 64)
 		updateID := id
-		isUpdate := validId > 0
+		isUpdate := id != "" && (parseErr != nil || validId > 0)
 		if !isUpdate {
 			// No valid /:id in the URL - check whether the body itself carries
 			// a valid primary key value and treat this as an update if so.
diff --git a/pkg/restheadspec/pk_update_integration_test.go b/pkg/restheadspec/pk_update_integration_test.go
index cba6e69..aed4eb6 100644
--- a/pkg/restheadspec/pk_update_integration_test.go
+++ b/pkg/restheadspec/pk_update_integration_test.go
@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 	"time"
 
@@ -123,3 +124,30 @@ func TestUpdateChangesPrimaryKeyWhenURLIDGiven(t *testing.T) {
 	require.NoError(t, db.QueryRow(`SELECT category FROM public.t_pkasset WHERE description = 'new-pk'`).Scan(&cat))
 	require.Equal(t, "cat5", cat)
 }
+
+// Exercises the real dispatch: a POST with a string id in the URL and a different
+// PK in the body must update the row identified by the URL id.
+func TestHandlePostWithStringURLIDChangesPrimaryKey(t *testing.T) {
+	db := setupPKTestDB(t)
+	reg := modelregistry.NewModelRegistry()
+	require.NoError(t, reg.RegisterModel("public.t_pkasset", pkAsset{}))
+	h := NewHandler(database.NewBunAdapter(bun.NewDB(db, pgdialect.New())), reg)
+
+	for _, method := range []string{http.MethodPost, http.MethodPut} {
+		t.Run(method, func(t *testing.T) {
+			from, to := "old-pk", "new-pk-"+method
+			if method == http.MethodPut {
+				from = "new-pk-" + http.MethodPost
+			}
+			req := httptest.NewRequest(method, "/public/t_pkasset/"+from,
+				strings.NewReader(`{"description":"`+to+`","category":"c"}`))
+			rec := httptest.NewRecorder()
+			w, r := common.WrapHTTPRequest(rec, req)
+			h.Handle(w, r, map[string]string{"schema": "public", "entity": "t_pkasset", "id": from})
+
+			require.Equal(t, http.StatusOK, rec.Code, rec.Body.String())
+			require.Equal(t, 0, countPK(t, db, from))
+			require.Equal(t, 1, countPK(t, db, to))
+		})
+	}
+}