diff --git a/lib/src/project.js b/lib/src/project.js
index 1d88da47..e5225299 100644
--- a/lib/src/project.js
+++ b/lib/src/project.js
@@ -10,7 +10,7 @@ const fields = [
   'silverFirArea',
   'relief',
 ];
-const fieldsConcat = ['slope', 'additional', 'silverFirArea', 'relief'];
+const secondaryFields = ['slope', 'additional', 'silverFirArea', 'relief'];
 const concat = (x, y) =>
   Array.from(new Set((x || []).concat(y))).sort((a, b) => a - b);
 
@@ -40,7 +40,7 @@ function projectionReducer(location, targetAltitudePointer, result) {
     const { field, value, values } = getField(fields[i], location);
     validate(field, value, values);
 
-    options[field] = fieldsConcat.includes(field)
+    options[field] = secondaryFields.includes(field)
       ? concat(options[field], Object.keys(projection))
       : options[field] || Object.keys(projection);
 
@@ -52,10 +52,9 @@ function projectionReducer(location, targetAltitudePointer, result) {
     } else if (valueNotInOptions(value, options[field])) {
       // Do not return location values if no projection was found.
       return { ...result, options };
-    } else if (fieldsConcat.includes(field)) {
+    } else if (secondaryFields.includes(field)) {
       // Fall back to first projection for secondary fields.
-      const first = Object.keys(projection)[0];
-      projection = projection[first];
+      projection = projection[options[field][0]];
     } else {
       // Location does not provide any more values for conditions.
       break;
@@ -117,7 +116,7 @@ function project(location = {}, targetAltitude, previousResult) {
     const transition = project(tl, targetAltitude);
     const tp = transition.projections || [];
     Object.entries(transition.options).forEach(([k, v]) => {
-      if (fieldsConcat.includes(k)) {
+      if (secondaryFields.includes(k)) {
         result.options[k] = concat(result.options[k], v);
       }
     });