diff --git a/public/app/core/components/TransformersUI/CalculateFieldTransformerEditor.tsx b/public/app/core/components/TransformersUI/CalculateFieldTransformerEditor.tsx
index 5f4b8b603c4..41f90e873f3 100644
--- a/public/app/core/components/TransformersUI/CalculateFieldTransformerEditor.tsx
+++ b/public/app/core/components/TransformersUI/CalculateFieldTransformerEditor.tsx
@@ -274,6 +274,7 @@ export class CalculateFieldTransformerEditor extends React.PureComponent<
         </div>
         <div className="gf-form">
           <Select
+            allowCustomValue={true}
             placeholder="Field or number"
             options={leftNames}
             className="min-width-18 gf-form-spacing"
@@ -289,6 +290,7 @@ export class CalculateFieldTransformerEditor extends React.PureComponent<
             menuPlacement="bottom"
           />
           <Select
+            allowCustomValue={true}
             placeholder="Field or number"
             className="min-width-10"
             options={rightNames}