diff --git a/rpmio/rpmio.c b/rpmio/rpmio.c
index 502a4b5..34e2e0f 100644
--- a/rpmio/rpmio.c
+++ b/rpmio/rpmio.c
@@ -3470,8 +3470,8 @@ fprintf(stderr, "*** Fdopen(%p,%s) %s\n", fd, fmode, fdbg(fd));
     if (stdio[0] == '\0')
 	return NULL;
     zstdio[0] = '\0';
-    strncat(zstdio, stdio, sizeof(zstdio) - strlen(zstdio));
-    strncat(zstdio, other, sizeof(zstdio) - strlen(zstdio));
+    strncat(zstdio, stdio, sizeof(zstdio) - 1 - strlen(zstdio));
+    strncat(zstdio, other, sizeof(zstdio) - 1 - strlen(zstdio));
 
     if (end == NULL && other[0] == '\0')
 	/*@-refcounttrans -retalias@*/ return fd; /*@=refcounttrans =retalias@*/