diff --git a/projects/ngx-formentry/src/form-entry/helpers/historical-expression-helper-service.ts b/projects/ngx-formentry/src/form-entry/helpers/historical-expression-helper-service.ts
index 05429b4d..68e72fa5 100644
--- a/projects/ngx-formentry/src/form-entry/helpers/historical-expression-helper-service.ts
+++ b/projects/ngx-formentry/src/form-entry/helpers/historical-expression-helper-service.ts
@@ -14,19 +14,22 @@ export class HistoricalHelperService {
 
   public evaluate(
     expr: string,
-    dataSources: any,
-    additionalScopevalues: any
-  ): any {
+    dataSources: Record<string, unknown>,
+    additionalScopeValues: Record<string, unknown> | null,
+  ) {
     const HD = new HistoricalEncounterDataService();
     HD.registerEncounters('prevEnc', dataSources['rawPrevEnc']);
-    const deps: any = {
-      HD: HD
-    };
+    if (dataSources.hasOwnProperty('rawPrevObs')) {
+      HD.registerEncounters('prevObs', dataSources['rawPrevObs']);
+    }
+
+    const deps = { HD };
 
-    if (additionalScopevalues) {
-      for (const o in additionalScopevalues) {
-        if (additionalScopevalues[o]) {
-          deps[o] = additionalScopevalues[o];
+    if (additionalScopeValues) {
+      for (const o in additionalScopeValues) {
+        const value = additionalScopeValues[o];
+        if (typeof value !== 'undefined' && value !== null) {
+          deps[o] = value;
         }
       }
     }