diff --git a/.run/silo--preprocessing.run.xml b/.run/silo--preprocessing.run.xml
new file mode 100644
index 000000000..3a9d5b112
--- /dev/null
+++ b/.run/silo--preprocessing.run.xml
@@ -0,0 +1,10 @@
+<component name="ProjectRunConfigurationManager">
+    <configuration default="false" name="silo --preprocessing" type="CMakeRunConfiguration" factoryName="Application" PROGRAM_PARAMS="--preprocessing --preprocessingConfig=testBaseData/test_preprocessing_config.yaml --databaseConfig=testBaseData/exampleDataset/database_config.yaml" REDIRECT_INPUT="false" ELEVATE="false" USE_EXTERNAL_CONSOLE="false" EMULATE_TERMINAL="false" WORKING_DIR="file://.." PASS_PARENT_ENVS_2="true" PROJECT_NAME="SILO" TARGET_NAME="siloApi" CONFIG_NAME="Debug" RUN_TARGET_PROJECT_NAME="SILO" RUN_TARGET_NAME="siloApi">
+        <envs>
+            <env name="SPDLOG_LEVEL" value="debug" />
+        </envs>
+        <method v="2">
+            <option name="com.jetbrains.cidr.execution.CidrBuildBeforeRunTaskProvider$BuildBeforeRunTask" enabled="true" />
+        </method>
+    </configuration>
+</component>
diff --git a/include/silo/storage/sequence_store.h b/include/silo/storage/sequence_store.h
index d5925663d..e3fe7a7c9 100644
--- a/include/silo/storage/sequence_store.h
+++ b/include/silo/storage/sequence_store.h
@@ -76,6 +76,12 @@ class SequenceStorePartition {
 
    void fillIndexes(const vector<optional<string>>& genomes);
 
+   void addSymbolsToPositions(
+      const size_t& position,
+      SymbolMap<SymbolType, vector<uint32_t>>& ids_per_symbol_for_current_position,
+      const size_t number_of_sequences
+   );
+
    void fillNBitmaps(const vector<optional<string>>& genomes);
 
   public:
diff --git a/src/silo/storage/sequence_store.cpp b/src/silo/storage/sequence_store.cpp
index 9314da7bc..1306aa9e3 100644
--- a/src/silo/storage/sequence_store.cpp
+++ b/src/silo/storage/sequence_store.cpp
@@ -170,25 +170,34 @@ void SequenceStorePartition<SymbolType>::fillIndexes(const vector<optional<strin
                   );
                }
             }
-            for (const auto& symbol : SymbolType::SYMBOLS) {
-               if (!ids_per_symbol_for_current_position.at(symbol).empty()) {
-                  positions[position].bitmaps[symbol].addMany(
-                     ids_per_symbol_for_current_position.at(symbol).size(),
-                     ids_per_symbol_for_current_position.at(symbol).data()
-                  );
-                  ids_per_symbol_for_current_position[symbol].clear();
-               }
-               if (symbol == positions[position].symbol_whose_bitmap_is_flipped) {
-                  positions[position].bitmaps[symbol].flip(
-                     sequence_count, sequence_count + number_of_sequences
-                  );
-               }
-            }
+            addSymbolsToPositions(position, ids_per_symbol_for_current_position, number_of_sequences);
          }
       }
    );
 }
 
+template <typename SymbolType>
+void SequenceStorePartition<SymbolType>::addSymbolsToPositions(
+   const size_t& position,
+   SymbolMap<SymbolType, vector<uint32_t>>& ids_per_symbol_for_current_position,
+   const size_t number_of_sequences
+) {
+   for (const auto& symbol : SymbolType::SYMBOLS) {
+      if (!ids_per_symbol_for_current_position.at(symbol).empty()) {
+         positions[position].bitmaps[symbol].addMany(
+            ids_per_symbol_for_current_position.at(symbol).size(),
+            ids_per_symbol_for_current_position.at(symbol).data()
+         );
+         ids_per_symbol_for_current_position[symbol].clear();
+      }
+      if (symbol == positions[position].symbol_whose_bitmap_is_flipped) {
+         positions[position].bitmaps[symbol].flip(
+            sequence_count, sequence_count + number_of_sequences
+         );
+      }
+   }
+}
+
 template <typename SymbolType>
 void SequenceStorePartition<SymbolType>::fillNBitmaps(const vector<optional<string>>& genomes) {
    const size_t genome_length = positions.size();
@@ -197,7 +206,6 @@ void SequenceStorePartition<SymbolType>::fillNBitmaps(const vector<optional<stri
 
    const tbb::blocked_range<size_t> range(0, genomes.size());
    tbb::parallel_for(range, [&](const decltype(range)& local) {
-      // For every symbol, calculate all sequence IDs that have that symbol at that position
       vector<uint32_t> positions_with_symbol_missing;
       for (size_t sequence_index = local.begin(); sequence_index != local.end(); ++sequence_index) {
          const auto& maybe_genome = genomes[sequence_index];