diff --git a/lib/selectors/calendar-selectors.js b/lib/selectors/calendar-selectors.js
--- a/lib/selectors/calendar-selectors.js
+++ b/lib/selectors/calendar-selectors.js
@@ -33,7 +33,10 @@
         continue;
       }
       if (result[threadID]) {
-        result[threadID].numVisibleEntries++;
+        result[threadID] = {
+          ...result[threadID],
+          numVisibleEntries: result[threadID].numVisibleEntries + 1,
+        };
       } else {
         result[threadID] = {
           threadInfo,
diff --git a/lib/types/filter-types.js b/lib/types/filter-types.js
--- a/lib/types/filter-types.js
+++ b/lib/types/filter-types.js
@@ -29,6 +29,6 @@
 };
 
 export type FilterThreadInfo = {
-  threadInfo: ThreadInfo,
-  numVisibleEntries: number,
+  +threadInfo: ThreadInfo,
+  +numVisibleEntries: number,
 };