diff --git a/configure b/configure
index e58846d5e2..e51a793987 100755
--- a/configure
+++ b/configure
@@ -125,7 +125,8 @@ cc_i386=i386-pc-linux-gnu-gcc
 libs_qga=""
 debug_info="yes"
 
-target_list=""
+# Don't accept a target_list environment variable.
+unset target_list
 
 # Default value for a variable defining feature "foo".
 #  * foo="no"  feature will only be used if --enable-foo arg is given
@@ -1288,7 +1289,7 @@ if ! "$python" -c 'import sys; sys.exit(sys.version_info < (2,4) or sys.version_
   exit 1
 fi
 
-if test -z "$target_list" ; then
+if test -z "${target_list+xxx}" ; then
     target_list="$default_target_list"
 else
     target_list=`echo "$target_list" | sed -e 's/,/ /g'`