diff --git a/bitblt.c b/bitblt.c
index 2701e32..c84b3e1 100644
--- a/bitblt.c
+++ b/bitblt.c
@@ -12,20 +12,23 @@ static inline u8 pixel_mask (x)
 #endif
 }
 
-static inline u32 rect_width (Rect r)
+static inline s32 rect_width (Rect r)
 {
   return (r.lower_right.x - r.upper_left.x);
 }
 
-static inline u32 rect_height (Rect r)
+static inline s32 rect_height (Rect r)
 {
   return (r.lower_right.y - r.upper_left.y);
 }
 
-Bitmap *create_bitmap (u32 width, u32 height)
+Bitmap *create_bitmap (s32 width, s32 height)
 {
   Bitmap *bitmap;
 
+  if ((width <= 0) || (height <= 0))
+    return (NULL);
+
   bitmap = calloc (1, sizeof (Bitmap));
   if (! bitmap)
     return (NULL);
diff --git a/bitblt.h b/bitblt.h
index a8f4c06..65d32aa 100644
--- a/bitblt.h
+++ b/bitblt.h
@@ -1,7 +1,7 @@
 typedef struct Point
 {
-  int x;
-  int y;
+  s32 x;
+  s32 y;
 } Point;
 
 typedef struct Rect
@@ -13,8 +13,8 @@ typedef struct Rect
 typedef struct Bitmap
 {
   u8 *bits;
-  u32 width;
-  u32 height;
+  s32 width;
+  s32 height;
   u32 rowbytes;
 } Bitmap;
 
@@ -35,7 +35,7 @@ typedef struct Bitmap
 #define ROT_270   (TRANSPOSE + FLIP_V)
 
 
-Bitmap *create_bitmap (u32 width, u32 height);
+Bitmap *create_bitmap (s32 width, s32 height);
 void free_bitmap (Bitmap *bitmap);
 boolean get_pixel (Bitmap *bitmap, Point coord);
 void set_pixel (Bitmap *bitmap, Point coord, boolean value);
diff --git a/type.h b/type.h
index d3e7f8d..edb8e8f 100644
--- a/type.h
+++ b/type.h
@@ -1,4 +1,7 @@
 typedef unsigned char  u8;
 typedef unsigned short u16;
 typedef unsigned int   u32;
+
+typedef int            s32;
+
 typedef int            boolean;