diff --git a/scp.c b/scp.c
index 56de4c7a..7c3b4257 100644
--- a/scp.c
+++ b/scp.c
@@ -15509,6 +15509,10 @@ int32 saved_switches = sim_switches & ~SWMASK ('T');
 t_stat stat = SCPE_OK;
 char gbuf[CBUFSIZE];
 
+if (sim_time != 0.0)
+    return sim_messagef (SCPE_UNK, "Library tests can not be performed after instructions have been executed.\n");
+detach_all (0, 0);                          /* Assure that all units are unattached */
+
 cptr = get_glyph (cptr, gbuf, 0);
 if (gbuf[0] == '\0')
     strcpy (gbuf, "ALL");