diff --git a/extras/cabi/README.md b/extras/cabi/README.md
index b6b8462..f68be19 100644
--- a/extras/cabi/README.md
+++ b/extras/cabi/README.md
@@ -77,10 +77,13 @@ given and a non-zero exit code will be returned.
 
 ## Input file decoding
 
-The input file should be a PNG file containing the image to be converted. The
-image will be translated to a raw array of 32 bit RGBA (Red, Green, Blue, Alpha)
-pixels internally before being processed to output. Ideally, pixels that are to
-be drawn (represented as a 1 in the image output) should be fully white.
+The input file should be a PNG file containing the image to be converted.
+The height of the image must be a multiple of 8 pixels (8, 16, 24, 32, ...).
+The width can be any size.
+
+The image will be translated to a raw array of 32 bit RGBA (Red, Green, Blue,
+Alpha) pixels internally before being processed to output. Ideally, pixels that
+are to be drawn (represented as a 1 in the image output) should be fully white.
 Non-drawn (0) pixels should be fully black. Pixels intended to be masked out of
 the image (represented as a 0 in both the image and mask output), should be
 fully transparent and their color doesn't matter.
diff --git a/extras/cabi/cabi.c b/extras/cabi/cabi.c
index ae88575..31f1bca 100644
--- a/extras/cabi/cabi.c
+++ b/extras/cabi/cabi.c
@@ -11,7 +11,7 @@ https://www.lexaloffle.com/bbs/?uid=1
 https://twitter.com/lexaloffle
 Contributed to Team A.R.G.
 
-Modifications by Scott Allen - July 2016
+Modifications by Scott Allen - July 2020, September 2020
 
 To the extent possible under law, the author(s) have dedicated all copyright
 and related and neighboring rights to this software to the public domain
@@ -332,6 +332,12 @@ int main(int argc, char **argv)
 		exit(result);
 	}
 
+	if (h % 8 != 0) {
+		printf("error 120: file %s: image height must be a multiple of 8 but is %u\n", argv[1], h);
+		free(bmp);
+		exit(120);
+	}
+
 	// generate sprite and mask
 
 	rawlen = w * (h+7) / 8;