diff --git a/configure b/configure
index 48d8df7677..a692695629 100755
--- a/configure
+++ b/configure
@@ -574,7 +574,7 @@ while [ $# -gt 0 ] ; do
 			set_variable crossToolsPrefix_$targetArch "$2"
 			targetArchs="$targetArchs $targetArch"
 			HAIKU_PACKAGING_ARCHS=
-			unknownArchIndex=$[$unknownArchIndex + 1]
+			unknownArchIndex=$(($unknownArchIndex + 1))
 			shift 2
 			;;
 		--distro-compatibility)