diff --git a/to-m4b.sh b/to-m4b.sh
index 540d0b0..bb79228 100755
--- a/to-m4b.sh
+++ b/to-m4b.sh
@@ -4,6 +4,8 @@ set -eufo pipefail
 
 # CONSTANTS
 declare -r script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# Use an array for the executable to avoid word-splitting issues
+declare -ra m4b_tool_bin=(nix run github:sandreas/m4b-tool#m4b-tool-libfdk --)
 
 # VARS
 LEADING_ZEROES=2
@@ -178,7 +180,7 @@ m4b-merge() {
   fi
   args+=("--output-file=${output_file}" -- "${source_dir}")
 
-  nix run github:sandreas/m4b-tool#m4b-tool-libfdk -- "${args[@]}"
+  "${m4b_tool_bin[@]}" "${args[@]}"
 }
 
 main() {