diff --git a/m4/include_next.m4 b/m4/include_next.m4
index b3c7849..d69fbce 100644
--- a/m4/include_next.m4
+++ b/m4/include_next.m4
@@ -87,14 +87,14 @@ dnl We intentionally avoid using AC_LANG_SOURCE here.
      rm -rf conftestd1a conftestd1b conftestd2
     ])
   PRAGMA_SYSTEM_HEADER=
-  if test $gl_cv_have_include_next = yes; then
+  if test x"$gl_cv_have_include_next" = x"yes"; then
     INCLUDE_NEXT=include_next
     INCLUDE_NEXT_AS_FIRST_DIRECTIVE=include_next
     if test -n "$GCC"; then
       PRAGMA_SYSTEM_HEADER='#pragma GCC system_header'
     fi
   else
-    if test $gl_cv_have_include_next = buggy; then
+    if test x"$gl_cv_have_include_next" = x"buggy"; then
       INCLUDE_NEXT=include
       INCLUDE_NEXT_AS_FIRST_DIRECTIVE=include_next
     else
@@ -178,7 +178,7 @@ AC_DEFUN([gl_NEXT_HEADERS_INTERNAL],
   m4_foreach_w([gl_HEADER_NAME], [$1],
     [AS_VAR_PUSHDEF([gl_next_header],
                     [gl_cv_next_]m4_defn([gl_HEADER_NAME]))
-     if test $gl_cv_have_include_next = yes; then
+     if test x"$gl_cv_have_include_next" = x"yes"; then
        AS_VAR_SET([gl_next_header], ['<'gl_HEADER_NAME'>'])
      else
        AC_CACHE_CHECK(
@@ -226,7 +226,7 @@ AC_DEFUN([gl_NEXT_HEADERS_INTERNAL],
      AC_SUBST(
        AS_TR_CPP([NEXT_]m4_defn([gl_HEADER_NAME])),
        [AS_VAR_GET([gl_next_header])])
-     if test $gl_cv_have_include_next = yes || test $gl_cv_have_include_next = buggy; then
+     if test x"$gl_cv_have_include_next" = x"yes" || test x"$gl_cv_have_include_next" = x"buggy"; then
        # INCLUDE_NEXT_AS_FIRST_DIRECTIVE='include_next'
        gl_next_as_first_directive='<'gl_HEADER_NAME'>'
      else