diff --git a/src/main/java/org/javarosa/core/model/ComparisonExpressionCacheFilterStrategy.java b/src/main/java/org/javarosa/core/model/ComparisonExpressionCacheFilterStrategy.java
index a1a1cb39a..d6d266eeb 100644
--- a/src/main/java/org/javarosa/core/model/ComparisonExpressionCacheFilterStrategy.java
+++ b/src/main/java/org/javarosa/core/model/ComparisonExpressionCacheFilterStrategy.java
@@ -10,9 +10,9 @@
 import org.javarosa.xpath.expr.XPathExpression;
 import org.jetbrains.annotations.NotNull;
 
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Supplier;
 
 /**
@@ -21,7 +21,7 @@
  */
 public class ComparisonExpressionCacheFilterStrategy implements FilterStrategy {
 
-    private final Map<String, List<TreeReference>> cachedEvaluations = new HashMap<>();
+    private final Map<String, List<TreeReference>> cachedEvaluations = new ConcurrentHashMap<>();
 
     @NotNull
     @Override
@@ -56,7 +56,10 @@ public List<TreeReference> filter(@NotNull DataInstance sourceInstance, @NotNull
         }
     }
 
-    private List<TreeReference> getCachedEvaluations(@NotNull Supplier<List<TreeReference>> next, String key) {
+    /**
+     * Synchronized to prevent two or more threads from modifying {@link #cachedEvaluations} at once
+     */
+    private synchronized List<TreeReference> getCachedEvaluations(@NotNull Supplier<List<TreeReference>> next, String key) {
         if (cachedEvaluations.containsKey(key)) {
             return cachedEvaluations.get(key);
         } else {