diff --git a/CHANGELOG.md b/CHANGELOG.md
index ad406eb0d70e49f2d1934b874fc2d3d7b5d17dd7..cf7c23d2d1831c7fa15946572fb22dd372b45a6a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@ All notable changes to Pantools will be documented in this file.
 - `pantools sequence_visualization` now requires the "sequence" rule to be set for unphased pangenomes (!226).
 - Fixed a number of bugs causing grouping results to slightly differ based on the order of genome or proteome input files (!223).
 - Fixed `pantools blast` for TBLASTN and TBLASTX where the wrong output file was written (!241).
+- Fixed the KMC command needed for `pantools add_genomes` (!243).
 
 ## [4.3.1] - 08-12-2023
 
diff --git a/src/main/java/nl/wur/bif/pantools/construction/index/IndexDatabase.java b/src/main/java/nl/wur/bif/pantools/construction/index/IndexDatabase.java
index 2d8e450e14d9af63ff38810560cb3a39a741b49e..50c343b9af4234df61ef4df78aa57b4dd42fc2ca 100755
--- a/src/main/java/nl/wur/bif/pantools/construction/index/IndexDatabase.java
+++ b/src/main/java/nl/wur/bif/pantools/construction/index/IndexDatabase.java
@@ -296,7 +296,7 @@ public final class IndexDatabase {
             // merge two indices
             Pantools.logger.debug("Merging two indices.");
 
-            final String merge_command = KMC + "_tools simple union " + index_path + "sorted " + index_path + "new " + index_path + "merged";
+            final String merge_command = KMC + "_tools simple " + index_path + "sorted " + index_path + "new union " + index_path + "merged";
             Pantools.logger.debug("KMC command: {}", merge_command);
 
             final String merge_output = runCommandWithOutput(merge_command);