Don't prefix tag names with 'v'. If you want that behaviour, include the 'v' on the command line manually.
diff --git a/git-flow-release b/git-flow-release
index 448a326..fa35e40 100644
--- a/git-flow-release
+++ b/git-flow-release
@@ -82,7 +82,7 @@
 	# merge into master
 	git checkout $MASTER_BRANCH
 	git merge --no-ff $BRANCH
-	git tag v$VERSION
+	git tag $VERSION
 
 	# merge into develop
 	git checkout $DEVELOP_BRANCH