diff --git a/man/mkcomposefs.md b/man/mkcomposefs.md
index b903e28..bb3d145 100644
--- a/man/mkcomposefs.md
+++ b/man/mkcomposefs.md
@@ -70,6 +70,9 @@ will be a mountable composefs image.
     actual image format version used will be adjusted upwards if that
     is beneficial for the image, up to the max version.
 
+**\-\-threads**
+:   Number of threads to be used to calculate the file digests and copy.
+
 # FORMAT VERSIONING
 
 Composefs images are binary reproduceable, meaning that for a given
diff --git a/tools/mkcomposefs.c b/tools/mkcomposefs.c
index 21b051b..e322956 100644
--- a/tools/mkcomposefs.c
+++ b/tools/mkcomposefs.c
@@ -1201,7 +1201,7 @@ static void usage(const char *argv0)
 		"  --from-file           The source is a dump file, not a directory\n"
 		"  --min-version=N       Use this minimal format version (default=%d)\n"
 		"  --max-version=N       Use this maxium format version (default=%d)\n"
-		"  --threads=N       	 Use this to calculate digest and copy files in threads (default=%d)\n",
+		"  --threads=N           Use this to calculate digest and copy files in threads (default=%d)\n",
 		bin, LCFS_DEFAULT_VERSION_MIN, LCFS_DEFAULT_VERSION_MAX, 1);
 }
 
@@ -1352,6 +1352,10 @@ int main(int argc, char **argv)
 				fprintf(stderr, "Invalid threads count %s\n", optarg);
 				exit(EXIT_FAILURE);
 			}
+			if (threads <= 0) {
+				fprintf(stderr, "Invalid threads count %d\n", threads);
+				exit(EXIT_FAILURE);
+			}
 			break;
 		case ':':
 			fprintf(stderr, "option needs a value\n");