diff --git a/git-am.sh b/git-am.sh
index 26ffe702e0..c132f50da5 100755
--- a/git-am.sh
+++ b/git-am.sh
@@ -205,7 +205,7 @@ check_patch_format () {
 			# and see if it looks like that they all begin with the
 			# header field names...
 			sed -n -e '/^$/q' -e '/^[ 	]/d' -e p "$1" |
-			egrep -v '^[A-Za-z]+(-[A-Za-z]+)*:' >/dev/null ||
+			LC_ALL=C egrep -v '^[!-9;-~]+:' >/dev/null ||
 			patch_format=mbox
 		fi
 	} < "$1" || clean_abort