diff --git a/packages/grafana-ui/src/components/GraphNG/utils.ts b/packages/grafana-ui/src/components/GraphNG/utils.ts
index c67e3d88b28..8d3b0c44751 100644
--- a/packages/grafana-ui/src/components/GraphNG/utils.ts
+++ b/packages/grafana-ui/src/components/GraphNG/utils.ts
@@ -2,38 +2,37 @@ import { XYFieldMatchers } from './types';
 import { ArrayVector, DataFrame, FieldType, outerJoinDataFrames } from '@grafana/data';
 import { nullToUndefThreshold } from './nullToUndefThreshold';
 
-function applySpanNullsThresholds(frames: DataFrame[]) {
-  for (const frame of frames) {
-    let refField = frame.fields.find((field) => field.type === FieldType.time); // this doesnt need to be time, just any numeric/asc join field
-    let refValues = refField?.values.toArray() as any[];
+// will mutate the DataFrame's fields' values
+function applySpanNullsThresholds(frame: DataFrame) {
+  let refField = frame.fields.find((field) => field.type === FieldType.time); // this doesnt need to be time, just any numeric/asc join field
+  let refValues = refField?.values.toArray() as any[];
 
-    for (let i = 0; i < frame.fields.length; i++) {
-      let field = frame.fields[i];
+  for (let i = 0; i < frame.fields.length; i++) {
+    let field = frame.fields[i];
 
-      if (field === refField) {
-        continue;
-      }
+    if (field === refField) {
+      continue;
+    }
 
-      if (field.type === FieldType.number) {
-        let spanNulls = field.config.custom?.spanNulls;
+    if (field.type === FieldType.number) {
+      let spanNulls = field.config.custom?.spanNulls;
 
-        if (typeof spanNulls === 'number') {
-          field.values = new ArrayVector(nullToUndefThreshold(refValues, field.values.toArray(), spanNulls));
-        }
+      if (typeof spanNulls === 'number') {
+        field.values = new ArrayVector(nullToUndefThreshold(refValues, field.values.toArray(), spanNulls));
       }
     }
   }
 
-  return frames;
+  return frame;
 }
 
 export function preparePlotFrame(frames: DataFrame[], dimFields: XYFieldMatchers) {
-  applySpanNullsThresholds(frames);
-
-  return outerJoinDataFrames({
+  let alignedFrame = outerJoinDataFrames({
     frames: frames,
     joinBy: dimFields.x,
     keep: dimFields.y,
     keepOriginIndices: true,
   });
+
+  return alignedFrame && applySpanNullsThresholds(alignedFrame);
 }