diff --git a/include/podio/RNTupleReader.h b/include/podio/RNTupleReader.h
index 97177ee71..d72cb9eb5 100644
--- a/include/podio/RNTupleReader.h
+++ b/include/podio/RNTupleReader.h
@@ -151,7 +151,7 @@ class RNTupleReader {
   // Map category to a vector that contains at how many entries each reader starts
   // For example, if we have 3 readers and the first one has 10 entries, the second one 20 and the third one 30
   // then the vector will be {0, 10, 30, 60}
-  std::unordered_map<std::string, std::vector<unsigned>> m_readerEntries;
+  std::unordered_map<std::string, std::vector<unsigned>> m_readerEntries{};
   std::unordered_map<std::string, unsigned> m_totalEntries{};
 
   struct CollectionInfo {