diff --git a/nullaway/src/main/java/com/uber/nullaway/ErrorBuilder.java b/nullaway/src/main/java/com/uber/nullaway/ErrorBuilder.java
index 74b2b1a613..94672d51eb 100755
--- a/nullaway/src/main/java/com/uber/nullaway/ErrorBuilder.java
+++ b/nullaway/src/main/java/com/uber/nullaway/ErrorBuilder.java
@@ -33,6 +33,7 @@
 
 import com.google.common.base.Joiner;
 import com.google.common.base.Preconditions;
+import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Lists;
@@ -360,7 +361,7 @@ void reportInitializerError(
       String message,
       VisitorState state,
       Description.Builder descriptionBuilder,
-      List<Symbol> nonNullFields) {
+      ImmutableList<Symbol> nonNullFields) {
     // Check needed here, despite check in hasPathSuppression because initialization
     // checking happens at the class-level (meaning state.getPath() might not include the
     // method itself).
diff --git a/nullaway/src/main/java/com/uber/nullaway/NullAway.java b/nullaway/src/main/java/com/uber/nullaway/NullAway.java
index 65ecccd5ad..1108fb8ad1 100644
--- a/nullaway/src/main/java/com/uber/nullaway/NullAway.java
+++ b/nullaway/src/main/java/com/uber/nullaway/NullAway.java
@@ -1517,12 +1517,16 @@ private void checkFieldInitialization(ClassTree tree, VisitorState state) {
       }
     }
     for (Element constructorElement : errorFieldsForInitializer.keySet()) {
-      List<Symbol> fieldSymbols =
-          errorFieldsForInitializer
-              .get(constructorElement)
-              .stream()
-              .map(element -> ASTHelpers.getSymbol(getTreesInstance(state).getTree(element)))
-              .collect(Collectors.toList());
+      ImmutableList<Symbol> fieldSymbols =
+          ImmutableList.<Symbol>builder()
+              .addAll(
+                  errorFieldsForInitializer
+                      .get(constructorElement)
+                      .stream()
+                      .map(
+                          element -> ASTHelpers.getSymbol(getTreesInstance(state).getTree(element)))
+                      .collect(Collectors.toList()))
+              .build();
       errorBuilder.reportInitializerError(
           (Symbol.MethodSymbol) constructorElement,
           errMsgForInitializer(errorFieldsForInitializer.get(constructorElement), state),