diff --git a/tools/in-build-gimp.sh b/tools/in-build-gimp.sh
index b4a3b47aab..b1254e7e98 100755
--- a/tools/in-build-gimp.sh
+++ b/tools/in-build-gimp.sh
@@ -2,7 +2,7 @@
 
 set -e
 
-export GIMP3_DIRECTORY=$(mktemp -d ${GIMP_GLOBAL_BUILD_ROOT}/.GIMP3-build-config-XXX)
+export GIMP3_DIRECTORY=$(mktemp -d ${GIMP_GLOBAL_BUILD_ROOT}/.GIMP3-build-config-XXXXXX)
 echo INFO: temporary GIMP configuration directory: $GIMP3_DIRECTORY
 
 if [ -n "$GIMP_TEMP_UPDATE_RPATH" ]; then
@@ -52,7 +52,7 @@ if [ -n "$GIMP3_DIRECTORY" ] && [ -d "$GIMP3_DIRECTORY" ] && [ -O "$GIMP3_DIRECT
     exit 1
   fi
   used_dir_prefix=$(realpath "$GIMP3_DIRECTORY")
-  used_dir_prefix=${used_dir_prefix%???}
+  used_dir_prefix=${used_dir_prefix%??????}
   tmpl_dir_prefix=$(realpath "$GIMP_GLOBAL_BUILD_ROOT")/.GIMP3-build-config-
   if [ "$used_dir_prefix" != "$tmpl_dir_prefix" ]; then
     echo "ERROR: \$GIMP3_DIRECTORY ($GIMP3_DIRECTORY) should be under the build directory with a specific prefix."