diff --git a/lib/src/project.js b/lib/src/project.js
index 7f0d0b83..e2f37a25 100644
--- a/lib/src/project.js
+++ b/lib/src/project.js
@@ -70,6 +70,7 @@ function projectionReducer(location, targetAltitudePointer, result) {
     .sort((a, b) => a.altitudePointer - b.altitudePointer)
     .reverse()
     .find(({ altitudePointer }) => altitudePointer <= targetAltitudePointer);
+
   if (projection && typeof projection.forestType === 'string') {
     const { altitudinalZone, forestType } = projection;
     result.projections.push({ altitudinalZone, forestType });
@@ -101,6 +102,16 @@ function project(location = {}, targetAltitude, previousResult) {
   if (previous && previous.altitudinalZone === last.altitudinalZone) {
     // Could not find projection to targetAltitude
     result.projections = [];
+
+    // Try to find projection using "unknown" for secondary fields.
+    for (let i = 0; i < secondaryFields.length; i += 1) {
+      const secondaryField = secondaryFields[i];
+      if (location[secondaryField] !== 'unknown') {
+        location[secondaryField] = 'unknown';
+        result = project(location, targetAltitude, result);
+        break;
+      }
+    }
   } else if (lastAltitudeIdx < targetAltitudeIdx) {
     result = project({ ...location, ...last }, targetAltitude, result);
   }
diff --git a/lib/test/project.test.js b/lib/test/project.test.js
index e44c9c33..c36e0fe6 100644
--- a/lib/test/project.test.js
+++ b/lib/test/project.test.js
@@ -156,6 +156,20 @@ describe('valid projections', () => {
     ).toStrictEqual([{ altitudinalZone: '80', forestType: '50*' }]);
   });
 
+  test('projection with fallback to unknown secondary field (silverFirArea)', () => {
+    expect(
+      project(
+        {
+          forestEcoregion: '2a',
+          altitudinalZone: '80',
+          forestType: '50*',
+          silverFirArea: '2',
+        },
+        '50',
+      ).projections.slice(-1)[0],
+    ).toStrictEqual({ altitudinalZone: '50', forestType: '12a' });
+  });
+
   test('projection with valid transitionForestType', () => {
     expect(
       project(