Index: wp-includes/rewrite.php
===================================================================
--- wp-includes/rewrite.php	(revision 15434)
+++ wp-includes/rewrite.php	(working copy)
@@ -1030,7 +1030,7 @@
 		if ( empty($this->category_base) )
 			$this->category_structure = trailingslashit( $this->front . 'category' );
 		else
-			$this->category_structure = trailingslashit( '/' . $this->root . $this->category_base );
+			$this->category_structure = trailingslashit( '/' . $this->root . rawurlencode( $this->category_base ) );
 
 		$this->category_structure .= '%category%';