diff --git a/util/mkdef.pl b/util/mkdef.pl
index 204a056a4..98cd12706 100755
--- a/util/mkdef.pl
+++ b/util/mkdef.pl
@@ -479,7 +479,7 @@ sub do_defs
 					push(@tag,$1);
 					$tag{$1}=-1;
 				}
-			} elsif (/^\#\s*ifdef\s+(.*)/) {
+			} elsif (/^\#\s*ifdef\s+(\S*)/) {
 				push(@tag,"-");
 				push(@tag,$1);
 				$tag{$1}=1;