diff --git a/backend/src/lib/cost-calc.js b/backend/src/lib/cost-calc.js
index d23d6ec..67c5e2c 100644
--- a/backend/src/lib/cost-calc.js
+++ b/backend/src/lib/cost-calc.js
@@ -136,7 +136,7 @@ export async function getPeriodConsumption(meterId, periodStart, periodEnd) {
   const endPoint = await interpolatedValueAtDate(meterId, periodEnd)
 
   if (!startPoint || !endPoint) {
-    const first = (await readingOnOrAfter(meterId, periodStart)) || (await readingOnOrBefore(meterId, periodStart))
+    const first = (await readingOnOrBefore(meterId, periodStart)) || (await readingOnOrAfter(meterId, periodStart))
     const last = await readingOnOrBefore(meterId, periodEnd)
     const hasData = !!(first && last && last.reading_date > first.reading_date)
     const basis = classifyBasis(hasData, periodStart, periodEnd, first, last)