diff --git a/src/tools/pkcs15-tool.c b/src/tools/pkcs15-tool.c
index 33161357..019ab8c0 100644
--- a/src/tools/pkcs15-tool.c
+++ b/src/tools/pkcs15-tool.c
@@ -1022,7 +1022,7 @@ static int read_ssh_key(void)
 		buf[1] = 0;
 		buf[2] = 0;
 		len = snprintf((char *) buf+4, 20, "ecdsa-sha2-nistp%d", n);
-		strncpy(alg, (char *) buf+4, 20);
+		strncpy(alg, (char *) buf+4, 19);
 		buf[3] = len;
 
 		len += 4;