diff --git a/src/controllers/api/crewShipFusionController.ts b/src/controllers/api/crewShipFusionController.ts
index be439789..c0981f4f 100644
--- a/src/controllers/api/crewShipFusionController.ts
+++ b/src/controllers/api/crewShipFusionController.ts
@@ -81,9 +81,11 @@ export const crewShipFusionController: RequestHandler = async (req, res) => {
             const newFval = (newPerc - rangeA[0]) / (rangeA[1] - rangeA[0]);
             buffA.Value = Math.trunc(newFval * 0x3fffffff);
         }
-        const useSuperiorSubroutine = tierA < tierB ? !payload.UseSubroutineA : payload.UseSubroutineA;
-        if (!useSuperiorSubroutine) {
-            fingerprint.SubroutineIndex = inferiorFingerprint.SubroutineIndex;
+        if (inferiorFingerprint.SubroutineIndex) {
+            const useSuperiorSubroutine = tierA < tierB ? !payload.UseSubroutineA : payload.UseSubroutineA;
+            if (!useSuperiorSubroutine) {
+                fingerprint.SubroutineIndex = inferiorFingerprint.SubroutineIndex;
+            }
         }
     }
     superiorItem.UpgradeFingerprint = JSON.stringify(fingerprint);