diff --git a/ui/src/views/Forecast.jsx b/ui/src/views/Forecast.jsx
index cd30251..f1a4635 100644
--- a/ui/src/views/Forecast.jsx
+++ b/ui/src/views/Forecast.jsx
@@ -1058,10 +1058,24 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
         .some(k => body[k] !== undefined)
       if (!has) { flash('Enter a target or increment', 'error'); return }
     }
-    if ((op === 'recode' || op === 'clone') && !Object.keys(body.set || {}).length) {
-      flash(op === 'recode' ? 'Enter at least one new dimension value' : 'Enter at least one override value', 'error')
+    // Recode with nothing set would rewrite rows as themselves. Clone would not:
+    // copying a segment forward in time changes the dates and the period
+    // dimensions, which is the whole point of cloning from prior year, so it needs
+    // no dimension override at all. It only needs to be doing *something* --
+    // an override, a shift, or a factor.
+    if (op === 'recode' && !Object.keys(body.set || {}).length) {
+      flash('Enter at least one new dimension value', 'error')
       return
     }
+    if (op === 'clone') {
+      const shifts  = body.date_offset && body.date_offset !== '0 days'
+      const scales  = body.scale != null && body.scale !== 1
+      const changes = Object.keys(body.set || {}).length > 0
+      if (!shifts && !scales && !changes) {
+        flash('A clone with no override, no date shift and a factor of 1 would just duplicate the rows', 'error')
+        return
+      }
+    }
 
     try {
       const res  = await fetch(`/api/versions/${versionId}/${op}`, {