diff --git a/util/mkdef.pl b/util/mkdef.pl
index 204a056a483c836f9e4b1a9a6ef67e73b2e0e74c..98cd12706cb4dd944b67004c6b7ff91ff5058c2f 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;