diff --git a/aegisub/m4macros/agi_find_libheader.m4 b/aegisub/m4macros/agi_find_libheader.m4
index 46bf1d82e..6e6334844 100644
--- a/aegisub/m4macros/agi_find_libheader.m4
+++ b/aegisub/m4macros/agi_find_libheader.m4
@@ -30,7 +30,7 @@ AC_DEFUN([AGI_FIND_HEADER],[
 AC_DEFUN([AGI_FIND_LIB],[
   aegisub_save_LDFLAGS="$LDFLAGS"
 
-  if test -n $3; then
+  if test -n "$3"; then
     LDDIR="-L$3"
   fi