diff --git a/snakePipes/shared/rules/createIndices.snakefile b/snakePipes/shared/rules/createIndices.snakefile
index d06a12853..e4f72080c 100755
--- a/snakePipes/shared/rules/createIndices.snakefile
+++ b/snakePipes/shared/rules/createIndices.snakefile
@@ -54,7 +54,7 @@ else:
         params:
             spikeinExt = spikeinExt
         shell: """
-            sed -r 's/\s+/{spikeinExt} /' {input} > {output}
+            sed -r 's/[[:space:]]+/{spikeinExt} /' {input} > {output}
         """
 
     rule createGenomeFasta: