diff --git a/sdk/install b/sdk/install
index ca5cb68..b1eb129 100755
--- a/sdk/install
+++ b/sdk/install
@@ -110,7 +110,7 @@ case $1 in
 					fi
 					dir=$(basename $(dirname $line))
 					#lower-case the directory part
-					outdir=${dir,,}
+					outdir=$(echo "$dir" | tr '[:upper:]' '[:lower:]')
 					if [[ $file == *.h ]] || [[ $file == *.i ]]; then
 						mkdir -p "$3/m68k-amigaos/include/$outdir"
 						echo cp "build/$2/$line" "$3/m68k-amigaos/include/$outdir/$file"