diff --git a/coders/sixel.c b/coders/sixel.c
index 6e9e006..1327309 100644
--- a/coders/sixel.c
+++ b/coders/sixel.c
@@ -672,8 +672,8 @@
     return x;
 }
 
-static MagickBooleanType sixel_encode_impl(unsigned char *pixels, int width,int height,
-                  unsigned char *palette, int ncolors, int keycolor,
+static MagickBooleanType sixel_encode_impl(unsigned char *pixels, size_t width,size_t height,
+                  unsigned char *palette, size_t ncolors, int keycolor,
                   sixel_output_t *context)
 {
 #define RelinquishNodesAndMap \