diff --git a/tinyprintf.c b/tinyprintf.c
index bb22700..f8f96da 100644
--- a/tinyprintf.c
+++ b/tinyprintf.c
@@ -70,14 +70,15 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  * Implementation
  */
 struct param {
-    char lz:1;          /**<  Leading zeros */
-    char alt:1;         /**<  alternate form */
-    char uc:1;          /**<  Upper case (for base16 only) */
-    char align_left:1;  /**<  0 == align right (default), 1 == align left */
-    unsigned int width; /**<  field width */
-    char sign;          /**<  The sign to display (if any) */
-    unsigned int base;  /**<  number base (e.g.: 8, 10, 16) */
-    char *bf;           /**<  Buffer to output */
+    unsigned char lz:1;  /**<  Leading zeros */
+    unsigned char alt:1; /**<  alternate form */
+    unsigned char uc:1;  /**<  Upper case (for base16 only) */
+    unsigned char align_left:1;
+                         /**<  0 == align right (default), 1 == align left */
+    unsigned int width;  /**<  field width */
+    char sign;           /**<  The sign to display (if any) */
+    unsigned int base;   /**<  number base (e.g.: 8, 10, 16) */
+    char *bf;            /**<  Buffer to output */
 };