diff --git a/backend/raster.c b/backend/raster.c
index 25fac7bb..80dca387 100644
--- a/backend/raster.c
+++ b/backend/raster.c
@@ -61,7 +61,7 @@ static void buffer_plot(struct zint_symbol *symbol, char *pixelbuf) {
     int fgred, fggrn, fgblu, bgred, bggrn, bgblu;
     int row, column, i;
 
-    symbol->bitmap = (char *) malloc(symbol->bitmap_width * symbol->bitmap_height * 3);
+    symbol->bitmap = (unsigned char *) malloc(symbol->bitmap_width * symbol->bitmap_height * 3);
 
     fgred = (16 * ctoi(symbol->fgcolour[0])) + ctoi(symbol->fgcolour[1]);
     fggrn = (16 * ctoi(symbol->fgcolour[2])) + ctoi(symbol->fgcolour[3]);
@@ -69,7 +69,7 @@ static void buffer_plot(struct zint_symbol *symbol, char *pixelbuf) {
     bgred = (16 * ctoi(symbol->bgcolour[0])) + ctoi(symbol->bgcolour[1]);
     bggrn = (16 * ctoi(symbol->bgcolour[2])) + ctoi(symbol->bgcolour[3]);
     bgblu = (16 * ctoi(symbol->bgcolour[4])) + ctoi(symbol->bgcolour[5]);
-
+    
     for (row = 0; row < symbol->bitmap_height; row++) {
         for (column = 0; column < symbol->bitmap_width; column++) {
             i = ((row * symbol->bitmap_width) + column) * 3;
diff --git a/backend/zint.h b/backend/zint.h
index 8686f318..c51c7c67 100644
--- a/backend/zint.h
+++ b/backend/zint.h
@@ -125,7 +125,7 @@ extern "C" {
         unsigned char encoded_data[200][143];
         int row_height[200]; /* Largest symbol is 189 x 189 Han Xin */
         char errtxt[100];
-        char *bitmap;
+        unsigned char *bitmap;
         int bitmap_width;
         int bitmap_height;
         unsigned int bitmap_byte_length;
diff --git a/docs/manual.txt b/docs/manual.txt
index 0b821b45..c3c7dc51 100644
--- a/docs/manual.txt
+++ b/docs/manual.txt
@@ -749,13 +749,13 @@ the example below where render_pixel() is assumed to be a function for drawing
 a pixel on the screen implemented by the external application:
 
 int row, col, i = 0;
-unsigned int red, blue, green;
+int red, blue, green;
 
 for (row = 0; row < my_symbol->bitmap_height; row++) {
      for (column = 0; col < my_symbol->bitmap_width; column++) {
-          red = my_symbol->bitmap[i];
-          green = my_symbol->bitmap[i + 1];
-          blue = my_symbol->bitmap[i + 2];
+          red = (int) my_symbol->bitmap[i];
+          green = (int) my_symbol->bitmap[i + 1];
+          blue = (int) my_symbol->bitmap[i + 2];
           render_pixel(row, column, red, green, blue);
           i += 3;
      }
@@ -826,7 +826,8 @@ row_height       | array of     | Representation of the       | (output only)
 errtxt           | character    | Error message in the event  | (output only)
                  |    string    |    that an error ocurred.   |
 bitmap           | pointer to   | Pointer to stored bitmap    | (output only)
-                 |    character |    image.                   |
+                 |    unsigned  |    image.                   |
+                 |    character |                             |
                  |    array     |                             |
 bitmap_width     | integer      | Width of stored bitmap      | (output only)
                  |              |    image (in pixels).       |